Most of the nominations that have been made so far are ones that I can agree with or am neutral towards, so I'll just go over the nominations that I disagree with.
Shellder: Remain A
Shellder has been a fantastic dedicated sweeper in LC for several years now. However, dedicated sweepers are a lot easier to come by than they were when Shellder was first moved to A+ back in ORAS - Shellder is no longer a uniquely powerful threat. It has been a mainstay of the upper tiers long enough for the metagame to completely adapt to it, and while a few trends still favour Shellder, the majority of metagame shifts hamper it. Webs tends to shut Shellder down, leaving it outsped by Elekid and Diglett and tied by 19 Speed Pokemon after setting up. Spikes users are difficult to come by outside of the momentum-draining Ferroseed, so Pokemon like slowfoo and Timburr are significantly more difficult to OHKO without Knock Off support. Vullaby is Nasty Plot as often as not, which can KO Shellder at +2; Scarf Doduo is able to outspeed and revenge-kill most Shellder that lack Ice Shard; and the decrease in Magnemite and Pawniard comes with a rise in Ferroseed, Thunderbolt Staryu, Slowpoke, and Mareanie. The most important change is the increase in Berry Juice for several glue Pokemon. Eviolite Ferroseed is KOed by Icicle Spear early game into +2 Icicle Spear late game, while Eviolite Croagunk and Alolan Grimer are brought into +2 Icicle Spear range after switching into Stealth Rock twice; the respective Berry Juice variants that have grown in popularity are all much harder to wear down.
1) Shellder is not significantly above other sweepers in sweeping ability.
- Shellder's bulk is only sufficient to set up once per match, against a moderate group of Pokemon, usually on the revenge-kill.
Shellder's resistances very rarely come into play, with Ice/Fire/Water users usually having better moves to hit Shellder with. If Shellder is relying on its resistances, it'll probably be against defensive Ice Beam Staryu (and risking one or two Scald burns), but defensive Ice Beam Staryu synergizes well with much more solid Shellder checks, like Ferroseed and Magnemite.
Most of the time, Shellder relies on its raw bulk to set up; the problem here is that it's only decently bulky. 20/18 physical bulk allows Shellder to survive the OHKO from just about any physical attack while setting up, but rarely a 2HKO if Shellder sets up Shell Smash on the first attack. It also doesn't leave Shellder with the bulk to withstand most 3HKOs, which means that Shellder tends to be left at fairly low health if it attempts to switch into an attack - even two of defensive Mienfoo's Drain Punch, one of the weakest physical attacks from a Pokemon faster than Shellder, is strong enough to put Shellder into range of Timburr's Mach Punch after Stealth Rock. On the other hand, Shellder's special bulk is abysmal, with the majority of offensive special attackers being able to OHKO it after Stealth Rock, from LO Gastly's Sludge Bomb to 15 SpA Staryu's Thunderbolt to Sashbra's Energy Ball. Pokemon slower than Shellder aren't any easier to set up on due to Shellder's defense drop after Shell Smash; Snubbull 2HKOes -1 Shellder with Play Rough if it chooses not to simply use Thunder Wave, and even Spritzee OHKOes -1 Shellder with Moonblast if Shellder has switched into Stealth Rock twice.
So Shellder's setup opportunities are mostly limited to physical attackers that it can OHKO after setting up, which is hard enough on its own if the opponent ensures that Shellder setup bait isn't out on a field when the Shellder user's Pokemon faints. On top of this, Shellder will almost always only be getting one shot at sweeping because its first setup attempt will leave it noticeably weakened. As a result, even teams lacking Shellder checks can work around it simply by limiting the number of Pokemon weak enough for Shellder to set up on and playing carefully with them.
- A large number of Pokemon are able to impede Shellder's sweep after it has set up.
While Shellder is a powerful sweeper when played well, it has its fair share of checks, even after setting up. Anything with at least 23 HP and 12 Def wielding an Eviolite, or 21 HP and 11 Def, can survive one of boosted Shellder's neutral attacks after Stealth Rock, assuming no high rolls/critical hits. About half of the Pokemon A- and over have notable sets fitting this description. Some of these Pokemon may be running frailer sets, but a heavily weakened Shellder can also be brought down by powerful priority or an especially fast Choice Scarf users; most teams will naturally carry around two Pokemon that can stop Shellder's sweep simply by surviving a boosted attack and hitting a damaged -1 Shellder. Diglett traps or heavily weakens some of these, but running both Shellder and Diglett, two Pokemon with very few switch-in opportunities, means you effectively only have four slots to cover the metagame defensively. Since Shellder can't VoltTurn into Diglett itself, further external support is required, and then there's the fact that some of Shellder's most solid and widespread checks, such as Slowpoke and Ferroseed, don't mind Diglett all that much. Wearing down these checks with Shellder itself is quite risky due to the fact that taking damage early game will frequently prevent Shellder from setting up late game, again due to its reliance on its only decent bulk to set up, so a significant portion of the team has to be dedicated solely towards a Shellder sweep. All this support may not even pay off, as especially Shellder-resilient teams can usually handle all this support without trouble seeing how Shellder usually only gets one shot at sweeping. What we end up with is a Pokemon whose soft checks are common enough, and hard checks solid enough, that it requires most of the team to back up its sweeping attempt, which puts it on par with Omanyte or Zigzagoon when it comes to counterplay - good enough that you can base a strong team around them, but not overly impressive.
That being said, Shellder is still generally more consistent than Omanyte or Zigzagoon because although it requires a comparable amount of support to eliminate checks, it's easier to set up, with Omanyte falling prey to coverage moves on birds, formerly its biggest chance to set up, that have become common to deal with Onix, and Zigzagoon being heavily reliant on Memento due to its below average bulk. Other sweepers that will usually find exactly one opportunity to sweep per match, such as Scraggy and Corphish, can be dealt with by a much greater variety of Choice Scarf users (though compared to Shellder, Scraggy in particular has fewer Pokemon that can immediately retaliate with a OHKO after surviving a boosted attack).
At the end of the day, Shellder is good enough in terms of both setup and sweeping ability to rank as arguably the best dedicated sweeper in LC, but as plenty of Pokemon rival or exceed it in either category, it's certainly not the best by a very large margin.
2) Shellder is unable to perform any role at a high level except for its standard sweeping set.
Shellder's only viable set is its Shell Smash set. While Shellder has had Choice Scarf and bulky Rapid Spin sets listed on its analysis in the past, they have become little more than novelty sets in the current metagame, seeing little to no use in high level play. Choice Scarf relies almost entirely on surprise value to be effective, rarely doing more than surprising an Abra looking to revenge-kill it. Though it has decent 2HKOing power, Shellder OHKOes very few Pokemon without a boost, failing to OHKO Eviolite Foongus after Stealth Rock. Along with a lack of VoltTurn, this makes it much less punishing than common Choice Scarf users, as even if Shellder predicts correctly, the opponent can simply switch to a resist afterwards, and the Shellder user will have lost momentum often without even netting a KO. Bulky Rapid Spin Shellder can threaten Pumpkaboo-Super more heavily than defensive Staryu and can switch into Doduo/Vullaby unlike Staryu/Sandshrew-Alola, but that's a steep price to pay for a slow and weak Pokemon without recovery or notable resistances, making it a momentum drain for offense and unable to fit into defensive cores for balance. Very few teams will appreciate bulky Shellder's miniscule advantages over Staryu.
On the Shell Smash set, Shellder has a variety of options to tailor its set for specific checks, most notably Ice Shard or a Jolly nature for Scarf Doduo and Scarf Gastly, Hydro Pump or Razor Shell for Mudbray and Magnemite, and Hidden Power Fighting for Pawniard and Magnemite. However, all of these only really help against one or two Pokemon out of Shellder's numerous checks, meaning they don't usually come into play. They also don't help to solve Shellder's core issues in limited setup opportunities throughout the match and being unable to muscle through any run-of-the-mill bulky attacker after a boost. Life Orb and Normalium help with the latter but leave it with even less chances to set up, necessitating Memento support, making them significantly less viable overall.
Contrast this with just about any other sweeper in the A and S ranks. The sweeping potential of Bulk Up Timburr and Nasty Plot Croagunk is secondary to their utility in strong priority, ability to check Scraggy and Steel-types, and Knock Off support. Timburr also has all-out attacker sets that are just as effective as its Bulk Up set, and an assortment of EV spreads to speed creep or survive specific special attacks. Croagunk has its own Bulk Up sweeper set and its classic mixed set. If they aren't acting as the dedicated sweeper, Doduo, Pawniard, Snivy, and Scraggy all have Knock Off and are strong enough to act as wall-breakers to clear the way for other sweepers; they also have Choice Scarf sets that are either very useful as revenge killers (Doduo, Pawniard) or have significant sweeping potential (Snivy, Scraggy). Torchic can Baton Pass its boosts to a teammate if it isn't in a position to sweep on its own.
The raw sweeping ability of Shell Smash Shellder is offset by how Shellder can do nothing else. When the opponent sees a Torchic or Scraggy in team preview, they have to ensure they have a plan for multiple sweeping sets until that Pokemon's set is revealed; when they see a Timburr or Vullaby, they have to play carefully with the sweeping set in mind, which allows the non-sweeping sets to more easily wall-break. The opponent can guess almost exactly what Shellder plans to do from the start of the match and drastically narrow down their plays accordingly.
3) Shellder is relatively difficult to fit onto a team.
Switching Shellder into an attack is usually a bad idea if you plan to sweep with Shellder, as explained earlier. Because of this, Shellder offers almost no defensive presence for your team; from a teambuilding perspective, you'll have to cover the metagame and prevent opposing sweeps with five Pokemon instead of six. No other sweeper in the A ranks except for Torchic fits this description. Croagunk remains a fantastic glue Pokemon on top of its sweeping potential - switching into Pawniard's Knock Off early on doesn't prevent it from checking Corphish or setting up on post-Sleep Clause Foongus later on. Snivy hard checks Ground-types, soft checks Water-types, and is the best Webs answer in the metagame. Even the frail Doduo discourages the likes of Pumpkaboo or Foongus from using Grass-type attacks and Spore throughout the match, and if it is running a Choice Scarf, it makes for a handy revenge-killer. Though you can run secondary sweepers alongside Shellder, you're still accepting that if you plan to use Shellder to good effect, you're primarily building around Shellder.
This ties in with Shellder's lack of versatility. If Shellder is running Ice Shard, it has access to priority that doesn't even come close to OHKOing Diglett. Otherwise, Shellder offers absolutely nothing outside of its potential to sweep. Snivy has Knock Off and Glare; Timburr has Knock Off and a reasonably strong Mach Punch; Carvanha can revenge-kill certain threats, has the immediate power to wall-break, and has a strong Aqua Jet; even Zigzagoon's Extremespeed acts as powerful priority. All of these Pokemon can support a teammate's sweeping attempt, check key threats, or revenge-kill a weakened sweeper even if they are unable to sweep on their own. Shellder cannot - if the opposing team is packing enough checks, you're playing close to a 5v6, plus a one time switch-in to Vullaby/Doduo/Torchic.
The combination of requiring a team built around it and still potentially being shut down means that a Shellder team tends to be quite matchup reliant. While teams that pack six Shellder checks and just win against Shellder builds aren't especially common, a negative matchup with, for example, a hard check and two soft checks, perhaps Ferroseed + Timburr + Berry Juice Chinchou, possesses a large advantage that is extremely difficult to build or play around. Of course, frailer teams that rely completely on preventing Shellder from setting up an fall prey to Memento into Shellder are just as common, but the fact is that Shellder is quite inconsistent. Pokemon in S and A+ are the faces of the metagame, the most splashable and consistent Pokemon available; Shellder is a much better fit for A or even A-.
Meowth: Remain C+
Meowth does have its unique strengths - if it was truly outclassed, it wouldn't be ranked at all - but there's no point in just listing what it can do without context. Meowth's capabilities have not changed for years, nor has its place in the metagame, and we already know what it can do; its C+ ranking has been assigned accordingly. If Meowth hasn't changed and the metagame hasn't changed, then the only way we have left to argue that Meowth should rise is if its original ranking outright wasn't fair, which we can do by comparing Meowth's ranking to the rankings of the Pokemon most similar to it.
Though none of them have Technician Fake Out + Feint, Meowth belongs to a select group of Pokemon that have absolutely no switch-in opportunities. There aren't very many of these in the upper ranks - even Doduo and Taillow can come in on Drilbur or Onix's Earthquake or Snivy's Leaf Storm / Knock Off, which are reasonably common. In comparison, Meowth comes in only on Ghost-type attacks, which are used on three relevant Pokemon and aren't the most spammable move for any of them; you can expect to almost never actually take advantage of this resistance. Simply including a Pokemon from this group on your team means that from a defensive standpoint, you are playing a 5v6. Seeing how difficult it is to check the entire relevant metagame even with six Pokemon, if you further handicap yourself defensively by including one of these, you're accepting that there will be multiple important threats that will get a KO against your team every time they come into play.
These Pokemon must have enormous offensive presence (or absolutely game-breaking support moves in Sticky Web or Aurora Veil, which Fake Out + Feint certainly doesn't qualify for) to even be worth considering. Being able to get a Pokemon in on a slow VoltTurn is rare and somewhat costly; to take advantage of slow VoltTurn, you have to get the slow VoltTurner in on something that is both unable to 2HKO it and is slower, which only applies to a few Pokemon per VoltTurner. Then, after the VoltTurner has taken a hit for free, the switch-in to the slow VoltTurner has to lose to the frail Pokemon being VoltTurned to, keeping in mind that your opponent can take this into account when deciding on their switch-in. Getting around relying on a slow VoltTurner by leading with the frail Pokemon can only happen once per match, on top of being highly predictable, so most of the time, these Pokemon will be coming into play through a revenge-kill. In this sort of scenario, it is important not to credit a KO solely to the offensive Pokemon in question, because that will get you an inflated sense of the threat's worth. The effort to get a frail Pokemon into play may be worth it if the frail Pokemon in question is Abra, which has the raw power to outright sweep once it comes in, or Diglett, which can guarantee a specific KO when the opportunity arises; more on this later. Just being able to reliably 2HKO switch-ins is nowhere near enough to warrant even a half-decent ranking; you'll be doing a one for one trade where the opponent comes out ahead in momentum, which is a net negative.
Meowth isn't even able to come close to guaranteeing a 2HKO after it comes in. Double-Edge allows bulkier Berry Juice users, scarfed Steel-types, and Weak Armor Pokemon to switch in and force it out; all of Meowth's other attacks allow a whole slew of threats a chance to get into play, as they do pitiful damage to moderately bulky Pokemon unless a coverage move is hitting it double super-effectively. Even a perfectly played Meowth has its hard checks in Snubbull, Ferroseed, Mudbray, and Kabuto. Any individual Meowth will actually have a lot more checks than that, though - after Fake Out + Feint, Meowth only has two remaining slots to cover the metagame. Out of its most common options, it has to choose from Double-Edge, Water Pulse, Knock Off, Aerial Ace; no matter its choices, Meowth will be walled by such a large portion of the metagame that its versatility becomes 4mss - it simply cannot consistently act as an offensive threat.
And if Meowth does get a KO, it's vulnerable to every trapper in the metagame. Diglett, Wynaut, and Trapinch all eliminate it without issue on the revenge-kill. Grimer-Alola KOes it with Pursuit after Stealth Rock + Double-Edge / Life Orb recoil, or after Stealth Rock if Meowth decides to switch. Scarf Pawniard can also trap a weakened Meowth with Pursuit. This means that you'll have to save Meowth for the late game if you want to utilize its revenge-killing potential against even a single sweeper.
With that in mind, let's go through each Pokemon that has a similar lack of defensive presence and is ranked above Meowth to see why they are far more viable. Surskit and Vulpix-Alola are strictly supportive so I've left them out.
Abra. With a set of Fake Out / Feint / Double-Edge / Water Pulse, Life Orb Meowth has the power to 2HKO the most important sets of all but 12 Pokemon from B- to S after Stealth Rock, out of 57 (though more can switch in and force it out at least once). With a set of Psychic / Dazzling Gleam / Energy Ball / Hidden Power Ground, Life Orb Abra is unable to
OHKO 13 of those after Stealth Rock. This difference is what makes Life Orb Abra the single most threatening Pokemon to stall and balance teams and one of the top webs abusers where Meowth falls flat in both aspects. If you predict correctly against Meowth, chances are you can force it out since you'll live the follow-up attack, invalidating all the effort needed to get Meowth into play and simultaeously gaining momentum. If you predict correctly against Abra, you're probably dead anyways. Then there's Sashbra, overall arguably the best Pokemon at 1v1s in the metagame, which is still far more difficult than Meowth to switch into, acts as a reliable safety net against a far greater number of threats, and can pose as the Life Orb set.
Offensive Staryu. While it isn't quite as capable of OHKOing the metagame as LO Abra is and so makes for a less effective webs abuser / wall-breaker, Staryu is still able to OHKO a much larger portion of the metagame than Meowth due to its stronger coverage moves. Furthermore, thanks to Analytic, Staryu is nearly as difficult to switch into as Abra is; pick Hydro Pump and three of [Thunderbolt, Ice Beam, Hidden Power Fire, Psychic] and you're looking at anywhere between two and four safe switchins from B- to S. Again, this means the difference between an incorrect prediction from Meowth resulting in some chip damage and lost momentum, and an incorrect prediction from Staryu resulting in the KO anyways. Offensive Eviolite Staryu has a similar number of switch-ins to Life Orb Staryu and can actually come in on a fair number of threats, though it outright OHKOes far fewer Pokemon.
Diglett. Diglett barely even belongs on this list because all of its variants can come in on stray Volt Switches and the Eviolite variant can come in on a bit more. Either way, there's a huge difference between how much you gain from a Meowth KO versus a Diglett KO. Even if you get Meowth in on a (likely weakened) team that lacks switchins, it'll probably KO a fodder Pokemon before being forced out again - the opponent chooses what faints, and you hope that Meowth's KO was worth more than the KO it took to get Meowth in. When you get Diglett in, you get to choose what faints with some minor constraints, often quickly paving the way for a teammate to sweep.
Offensive Torchic / Carvanha. As I mentioned earlier, the main reason why simply 2HKOing the metagame is not enough to warrant the use of a defensive deadweight, including Meowth, is that after it gets a kill, it is immediately forced out; so you sack one Pokemon to get the defensive deadweight in, get one kill with the defensive deadweight, and then you're forced out again, but the opponent gets to choose what to send in against Meowth, allowing them the upper hand in momentum and ultimately the advantage. That being said, this loss in momentum isn't very punishing if you're being forced out by a defensive Pokemon that doesn't hit very hard - if they send Spritzee in to revenge-kill your defensive deadweight, you can swap out to one of your many Spritzee checks to gain the upper hand again, forcing them to make a risky double switch as you go to your Spritzee check if they want to maintain momentum. On the other hand, you definitely don't want to be letting a powerful offensive threat into play for free - allowing a Pawniard or VoltTurn user in for free guarantees that the opponent will be able to maintain their momentum. Torchic and Carvanha are slightly more powerful than Meowth, and more importantly, circumvent the issue of letting offensive Pokemon in on the revenge-kill thanks to a combination of Speed Boost, power, and coverage, allowing them to outspeed and OHKO traditional revenge-killers. Meowth lacks the power to immediately threaten anything faster than it, as well as some things slower than it; after grabbing a 2HKO with Double-Edge, everything from Pawniard to Scarf Mienfoo to Elekid fires off a strong STAB attack or VoltTurn with impunity on Meowth or its switch-in. Torchic and Carvanha also have significant cleaning potential (and in Torchic's case, the potential to aid a teammate in cleaning with Baton Pass), which Meowth can't come close to doing most of the time unless the opposing team is within range of being swept by a Feint that's about as strong as Timburr's unboosted Mach Punch. This generally makes them far better anti-offense Pokemon than Meowth.
Elekid. While Elekid also only has the power to 2HKO most switchins like Meowth, its fast Volt Switch ensures that it is able to take full advantage of an opening much more consistently than Meowth is. On top of that, a one point difference in Speed means a lot in LC. Where Meowth has to rely on weak priority or a tie to take on Abra and Staryu, Elekid outspeeds them and pressures the opponent with a powerful Volt Switch, simultaneously doing a fair amount of chip damage and maintaining momentum. The closest equivalent Meowth has to this is a U-turn that's so pitifully weak that it's honestly not worth using at all. So once again, Meowth ends up in a position where an incorrect prediction means the attempt to get it into play was wasted, while Elekid doesn't have to worry about predictions to begin with when it can just use its mostly risk-free Volt Switch in a metagame where Chinchou isn't as common as it once was, following up with a Hidden Power Grass if Onix or Diglett chooses to switch in. In addition, unlike Meowth, Elekid has a defined niche on a major archetype, in this case webs, as a reliable offensive check to birds, courtesy of having STAB moves that actually provide coverage.
Aipom. Aipom is not the same as Meowth because Aipom is better than Meowth. Similarly to Abra or Staryu, it's a matter of how much it's worth to get Meowth or Aipom into play - Meowth has to predict perfectly to get a KO to even begin to make up for the opportunity cost, Aipom does not. Aipom effortlessly muscles through bulky Pokemon like Mareanie and Timburr with Fake Out + Fury Swipes where Meowth needs to 3HKO itself with Double-Edge to hope to 2HKO them. Aipom's decent bulk (for its power and speed) also makes it the far better 1v1 Pokemon and all around much more difficult to force out; after Stealth Rock and a round of Life Orb recoil, Aipom still has the bulk to tank 18 Atk Iron Fist Timburr's Mach Punch or make revenge-killing it with LO Diglett a risky task. Once Aipom comes in, you can expect it to get a KO; Meowth simply can't hold a candle to Aipom when it comes to actually being a threat.
Zigzagoon. Suppose a match between two offensive teams has gone on for about a dozen turns, and both teams are slightly weakened. If Meowth finds the opening to come into play, it obtains a single KO before immediately being forced out. If Zigzagoon gets into play, it defeats the opponent.
Bunnelby. Outside of Meowth's Fake Out + Feint combo, Scarf Bunnelby is, for the most part, a better version of Meowth. It's faster, stronger, and doesn't wipe itself out on its main STAB attack. Despite carrying a scarf, Bunnelby is arguably less reliant on prediction than Meowth is, as its coverage moves pack enough of a punch to not be easy switch-in opportunities for most of the metagame, and it can take advantage a fairly powerful U-turn with little risk. A few Pokemon can take advantage of immunities and resistances to set up on a locked in Bunnelby, but at the same time, moderately bulky sweepers like Dragon Dance Scraggy can set up on Meowth where they would be heavily damaged by Bunnelby. Life Orb Bunnelby is the first Pokemon on this list that doesn't completely overshadow Meowth as a threat since Meowth is so much faster. However, Life Orb Bunnelby has a defined niche as a powerful wall-breaker for Webs teams, whereas Meowth doesn't fit in very well on any archetype in particular.
Now that we have hopefully established that Meowth is a terrible, terrible offensive Pokemon in comparison to the rest of the metagame, we can look at the utility Fake Out + Feint offers. Seeing how much power and momentum you give up by choosing Meowth over another frail offensive Pokemon, Meowth would have to have enormous utility to make up for turning the game into a 5v6 defensively. To put it bluntly, Fake Out + Feint isn't that good. It's a nice perk that comes in handy for specific situations, but cannot be relied on to any significant extent. It's strong enough to barely 4HKO (2HKO in practice) most Eviolite sweepers that don't resist it, like Dragon Dance Scraggy, and only the frailest Pokemon are threatened by the 2HKO (OHKO in practice). Certain sweepers, like Tirtouga, take laughable damage from Meowth's attacks, while others, like Scraggy and Timburr, simply recover the damage with Drain Punch. Meowth can force out a few fast and frail threats like Scarf Mienfoo and Abra, but only on the revenge-kill, and many of these Pokemon really aren't hampered whatsoever by switching, nor is the metagame short of Pokemon able to tank Fake Out, so the Meowth user still usually ends up on the losing end.
So it comes down to Meowth acting as a safety net against the likes of Weak Armor Vullaby, Zigzagoon, Carvanha, Life Orb Torchic, and Shellder, but Meowth is an extremely poor answer to all of these except for Protect-less Zigzagoon. To begin with, Meowth is unable to safely switch in on any of them except for their setup moves, which already discounts it from being anything more than a soft check. Even if Meowth does come in on Vullaby's Nasty Plot, Fake Out either activates Berry Juice if Stealth Rock is up or simply pops Weak Armor if Vullaby is healthy. Meowth will then have to switch out, which could end disastrously if Vullaby is carrying Substitute. But even if Meowth plays its hand perfectly, it will have to sack a Pokemon to come in on Vullaby, and then Vullaby will likely still be healthy enough to switch out of the obvious revenge-killing attempt to pose as a significant threat later on. Shellder is a similar case to Vullaby, although it does not usually carry Substitute. Keep in mind that the sack needed for Meowth to revenge-kill Vullaby or Shellder is especially costly when you consider how Meowth isn't offering any defensive presence for the team - by relying on Meowth to any significant extent, you're giving up two switch-ins to various metagame threats to deal with what Meowth is supposed to handle.
Meowth needs both Fake Out and Feint to knock out a relatively healthy Carvanha or Torchic, but they can Protect on Fake Out, and for every correct prediction the Carvanha or Torchic user makes, they get an extra KO at no cost, assuming Meowth is its team's best answer. Meowth can use Feint to beat the Protect, but it needs Carvanha or Torchic to have taken five rounds of Life Orb recoil and Stealth Rock chip before it KOes, which is likely far more than a team relying on Meowth to check them can afford, so if Carvanha / Torchic predicts the Feint and simply attacks, Meowth is a goner. This is all taking into account how Meowth can't even safely switch in on Carvanha or Torchic - it needs to win a 50/50 to even revenge-kill them. A similar case to Carvanha / Torchic plays out against Protect Zigzagoon, with the additional obstacle of Meowth's Fake Out + Feint not OHKOing from full if Zigzagoon manages to remain healthy after setup. Even against the Pokemon Meowth is supposed to act as a safety net against, Meowth either relies on an extra sack or a 50/50 to revenge-kill them, which Meowth's team may not be able to afford.
In other words, Meowth is so dreadful as an immediate offensive threat and a safety net that it struggles to qualify as either.
Now I would like to compare Meowth to the C+ and B- rankings and what they represent. First, let's look at Magby, which will likely be dropping to C+ soon, and Tyrunt, a Pokemon that is already in C+. Both of these Pokemon and Meowth appear to have useful strengths at first, but when you compare them to the rest of the metagame, they simply don't seem to be worth using except on the most niche of teams or for novelty value. Despite having different checks than Shellder or Zigzagoon, Magby ultimately sees little use over more prominent sweepers because it's so much more costly and difficult to support; Tyrunt lacks Onix's initial Speed, Omanyte and Onix's Weak Armor, Onix and Tirtouga's bulk, and Omanyte and Tirtouga's Shell Smash to compete against any of them with its Stealth Rock and Dragon Dance sets; Meowth is underwhelming at everything it wants to do, not to mention outshined by Sashbra as a wall-breaker safety net, which is a position that most teams can really only afford to give up one slot for if any, for an even slightly Shellder-resilient team.
Then we have Salandit, a C+ Pokemon that could potentially move up to B- in the near future, and Hippopotas, a Pokemon that is firmly a resident of B- until Magnemite becomes a top threat again. Salandit is a great Snivy check that can also get on the nerves of Foongus, Fairy-types, and the omnipresent Fighting-types. It can set up Nasty Plot and utilize Z-moves quite well to muscle through bulkier Pokemon, and Fire/Poison/Grass coverage is difficult to switch into for most teams. Though it is outsped by Abra and Staryu, it makes for a good choice on webs teams that are looking for a Snivy answer, as it is much better able to threaten a sweep than the likes of Ponyta, Deerling, or Blitzle. Hippopotas is, bar none, the most reliable answer to Magnemite and Elekid; it also shuts down several other important Pokemon in Croagunk, Grimer-Alola, and Onix, checks Pawniard, and phazes Vullaby, Scraggy, and Timburr. It's quite easy to come up with a solid, defensively oriented team that needs Hippopotas's specific strengths more than any other Pokemon, where Hippopotas's access to Stealth Rock, phazing, and reliable recovery is more useful than Mudbray's offensive presence. Both Salandit and Hippopotas have defined niches on defined archetypes. Meowth does not, because Meowth is not good at what it does. On almost every team that uses Meowth, you'll probably be able to replace it with no detriment, with very few exceptions.
In short, Meowth is an unreliable and gimmicky choice that is often indicative of lazily built, suboptimal teams. Putting it anywhere above C+ would imply that it has a defined niche on consistently competitive teams when it does not.
Spinarak: Remain B-
Sticky Web is certainly the best it's ever been in LC, but this is because some of its most prominent abusers, such as Gastly, offensive Croagunk, and most importantly, Life Orb Abra, happen to be in a very good spot for metagame matchups. One thing all of these Pokemon have in common is that they need Webs to be up as soon as possible to best threaten the opponent, with Abra and Gastly relying on it to not be forced out by various Choice Scarf users or trapped by Scarf Pawniard, and Abra and Croagunk needing it to get around Diglett. As the fastest Sticky Web user, Surskit is ideal for getting webs up as fast as possible, whereas Spinarak will often be forced to avoid the lead matchup out of fear of a Taunt or Rock Blast user and have to find the opportunity later in the match. Another thing these webs abusers have in common is that they don't really mind Foongus or Fighting-types, with Croagunk even using them as setup bait for Nasty Plot. It's unlikely that the entire webs team will be resilient to Foongus and Fighting-types - in particular, Eviolite Snivy is a common webs abuser that lets Foongus in quite easily - but the hyper offensive nature of these teams means that they don't mind sacking a Pokemon too much. So Spinarak's ability to switch in on key threats to set up Sticky Web doesn't benefit the current most prominent Sticky Web builds very much, or at least, Surskit's ability to set up webs right off the bat is far more useful.
Spinarak's niche is primarily on more balance-oriented webs teams that don't mind waiting until mid-game to get Sticky Web up; they are much more appreciative of having a solid switch-in to Foongus, Fighting-types, and Fairy-types to ease pressure off the rest of the team, a Pokemon that can repeatedly set up webs when they are unable to pressure hazard removers, and potentially a Toxic Spikes user. However, these teams have not grown in prominence the way hyper offensive webs has, so while webs as a whole has improved dramatically, Spinarak has not.
Snivy: Remain A
Web is good, but Snivy better.
Zigzagoon: Remain B+
Zigzagoon teams can be very strong when played well, but you really can't claim that it doesn't need much support - it's the poster child of needing a lot of support. That's why you need an entire team backing it up. Without Stealth Rock, a large portion of Eviolite-holding Pokemon will be able to survive its Extremespeed and revenge-kill it, including Timburr, Vullaby, Foongus, and Alolan Grimer, turning the game into a 5v6 when factoring in how Zigzagoon will be coming in on the revenge-kill. Without Knock Off support, various exceptionally bulky Pokemon such as Spritzee and physically defensive Slowpoke can revenge-kill a weakened Zigzagoon, turning the game into a 5v6 in the same manner. Without Pursuit support, Ghost-types and Sashbra shut its sweeping attempt down while still wall-breaking early game, turning the game into a 5v6. Without Memento support, Zigzagoon has trouble setting up in a metagame where every Pokemon in A+ and S except for Nasty Plot Vullaby, Berry Juice Grimer, and Eviolite Staryu completely deny its setup - but even then, Vullaby can flinch, bj Grimer can poison or high roll with Gunk Shot, and Staryu can burn with Scald. All this support points toward an even more extreme version of what I described with Shellder. You're gambling on matchup; if your opponent happens to have more than a couple of Zigzagoon checks, it'll be tough to get all of them onto the field to sufficiently weaken or trap; you're most likely out of luck.
Now, if Zigzagoon's checks were uncommon enough, Zigzagoon might still be one of the top sweepers, but there is a reason that Zigzagoon teams see far less use than Shellder or Scraggy teams. Even after finding the opportunity to set up, Counter Sashbra, Onix, Gastly, Ferroseed, and Pawniard are all very prominent Pokemon that many teams will carry more than one of and can completely shut Zigzagoon down. Fightspam and webs are two of the most common archetypes, and both allow Zigzagoon little to no setup opportunities, even when Zigzagoon is used with Memento. Furthermore, fightspam is often run alongside Onix, and webs will be running a Ghost-type that isn't pressured into being sent in early game due to most Zigzagoon teams being unable to fit a spinner in. Zigzagoon teams will always have their place in the metagame, but the current metagame isn't any friendlier to it than previous metagames.
Goldeen: Remain C
Between Knock Off, Waterfall, Drill Run, and Z-Bounce / Megahorn, Goldeen is actually pretty rough to switch into if you don't have Ferroseed or, in the case of Megahorn Goldeen, Foongus. Lightning Rod allows Goldeen to hard check Chinchou and Magnemite and soft check Elekid and Slowpoke. Comparably reliable checks to Chinchou (Foongus, Ferroseed, RestTalk Chinchou, Lileep) have much less offensive presence than Goldeen, so if you're running an exceptionally Chinchou-weak team on hyper offense, require a fairly threatening Knock Off user and general attacker, and already have a hazard setter over Ferroseed / Lileep, Goldeen is barely justifiable. That being said, Goldeen is still pretty bad; most of the time, you'll be better off just running Snivy and risking a poor matchup against Ice Beam Chinchou. But since there are at least a small number of competitive teams that genuinely appreciate Goldeen's advantages as much as its competition's, I feel that Goldeen ranks above the Pokemon in C-, which tend to find spots on teams almost solely for their novelty value.
Tirtouga: Remain B
Tirtouga has never been as potent a sweeper as Shellder or Omanyte. Its awful speed tier has always left it too vulnerable to Choice Scarf users to effectively clean; it will usually need two Shell Smashes to have a reasonable shot at sweeping past them, but this turns Tirtouga into easy pickings for priority users. Since late XY, Tirtouga has had to deal with a large portion of the top Pokemon in any given metagame being able to check one or more of its Shell Smash sets, so it wouldn't make sense to drop Tirtouga now on those grounds alone.
The reason Tirtouga has remained a notable threat despite its limited sweeping potential is how easily it comes in. Solid Rock Tirtouga avoids the OHKO from pretty much everything that isn't a strong Grass-type attack, including Reckless Scarffoo's High Jump Kick if Eviolite is run, and unlike Shellder, it actually has usable resistances. This ridiculous bulk grants Tirtouga an enormous amount of setup opportunities, and makes setting up twice a real possibility. More importantly, despite Tirtouga's issues with sweeping, it is able to pull its weight all the same. Eviolite Tirtouga is the metagame's best Doduo check and can survive Torchic's LO HP Grass after Stealth Rock, while Berry Juice Tirtouga can avoid the 2HKOs from Vullaby and Taillow's HP Grass from full. Tirtouga is certainly a threat once it comes in; Stone Edge and Zen Headbutt are somewhat difficult to switch into even before setup, Aqua Jet is reasonably strong for a priority move, and you can sneak in a Knock Off or Stealth Rock if you're willing to further sacrifice its sweeping potential. Tirtouga's bulk allows it to come in and whittle down its own checks early game relatively safely, before setting up Shell Smash mid or late game to punch huge holes in the opposition. And of course, if the enemy leaves Tirtouga alone for too long, it'll get enough Shell Smashes up to sweep.
Tirtouga's ability to come into play as a bulky tank and then quickly transform into an offensive threat with real sweeping potential, coupled with its versatility, forces the enemy to stay on their toes, and gives Tirtouga plenty of chances to weaken its checks; this makes it the perfect partner to a dedicated sweeper that shares its checks. As a result, Tirtouga is one of the faces of fishspam cores alongside Corphish, LO Staryu, and Carvanha. Tirtouga has far more opportunities to get into play than these other fishspam Pokemon, and is the one best equipped to both lure in and confront Croagunk in particular. All of the main fishspam threats are dangerous wall-breakers as well, allowing for some flexibility on which member of the core will break and which will sweep depending on the match. B- currently belongs to Pokemon that are secondary choices in standard archetypes, or primary choices in secondary archetypes. Though the popularity of Foongus, Croagunk, and Ferroseed makes fishspam less tempting than it has been in the past, it is still very much a prominent and proven strategy; the rise of firespam may be enough to bring fishspam back into the limelight. Tirtouga is an integral member of an archetype that is still considered standard, and is honestly closer to B+ than it is to B-.
Mantyke: Remain C-
Offensive Mantyke isn't really worth using. It has its strengths, and perhaps once in a while you'll be able to pull off a nice sweep with it. Offensive Mantyke simply does not have a real competitive niche; there is no reason to include it over one of the numerous better sweepers that are easier to build around and have more sweeping potential. It's moderately powerful and quite fast in the rain, but pales in comparison to Shell Smashers, Scraggy, and other top, high, or even mid tier sweepers after setup in terms of damage, and still isn't faster than + Speed Shell Smashers - the fastest Scarf users, Elekid and Diglett, can still take it out. Mantyke is shut down by more bulky Pokemon than any remotely notable sweeper ranked C or over; basically everything that resists Water bar frail Grass-types and some that do not can stop its sweep. It can set up on a few Pokemon here and there, but more prominent sweepers that don't carry a Stealth Rock weakness, have a workable defense stat, and have enough power to viably run Eviolite set up on much more. The inability to run a defensive item is key here; Mantyke absolutely needs to run Life Orb or Waterium Z, or else Berry Juice Vullaby survives Rain-boosted Hydro Pump from full, and it can't OHKO even Tauntfoo after a layer of Spikes with Air Slash. This makes Mantyke's Special Defense stat much less impressive, and mostly negates its otherwise decent set of resistances; 16 Atk Iron Fist Timburr has a good chance to KO LO Mantyke with Knock Off into Mach Punch after Stealth Rock, and LO Abra OHKOes Mantyke with Psychic after Stealth Rock more often than not. Although there will be scenarios where Mantyke sweeps, chances are that any half-decent sweeper would have been able to clean far more easily, and maybe without relying on Hydro Pump - you're handicapping yourself by running Mantyke instead. Every competitively designed team (this excludes Rain) where offensive Mantyke is run can be improved by replacing Mantyke with a sweeper that is not Mantyke.
RestTalk Mantyke is undebatably the absolute worst set in the metagame. It does nothing once it comes into play except attempt to spread Scald burns, and it's not even that good at spreading them because it's a horrible Pokemon that can only come in on a bunch of defensive Pokemon that most things can come in on anyways and hopefully do more than attempt to spread Scald burns in the process. Defensive Mantyke is thoroughly outclassed by defensive Ducklett, which at least gets Roost; relying on RestTalk is generally a horrible idea when you're weak to Stealth Rock, so defensive Mantyke just gets used as fodder most of the time, except it's not even good at being fodder because so many bulkier setup sweepers can use it as setup bait when it's so weak. Also, Ducklett gets Defog and Hurricane, allowing it to support the team or somewhat threaten frailer Pokemon if they miss their attacks or something. Defensive Mantyke, on the other hand, requires Stealth Rock to 2HKO Foongus with Air Slash - Foongus isn't even that bulky for a defensive Pokemon. And really, compared to defensive Mantyke, Foongus and its Sludge Bomb may as well be the most threatening offensive Pokemon in the game, because again, Mantyke is piss weak. You'd think that the sweet typing and SpD should let it check at least some cool offensive threats, but unfortunately, RestTalk Mantyke is full of utter disappointment in every respect. HJK -> Knock Off -> HJK from defensive min Attack Mienfoo KOes Mantyke after Stealth Rock, every single Mienfoo bar Baton Pass outspeeds Mantyke, and every single Eviolite Mienfoo variant easily avoids the OHKO from Air Slash (even Scarffoo lives from full), so it doesn't even check Eviolite Mienfoo. Mantyke actually doesn't really check any Fighting-type except for Regenerator Scarffoo and Bulk Up Timburr. Eviolite Snivy, LO Gastly, and LO Abra 2HKO Mantyke after Stealth Rock, while Swords Dance Corphish and Nasty Plot Vullaby set up in Mantyke's face, and Mantyke, being exceptionally wimpy for a piece of garbage, doesn't come close to OHKOing them, so the excellent special wall Mantyke loses to more special attackers than not. Even Sashbra has a roughly 45% chance of defeating a Mantyke switching in with full health when factoring in Psychic crits/max rolls/drops, but realistically, Mantyke is getting chunked by Stealth Rock, and then Abra is going to switch out to whatever because Mantyke is weak and more things wall it than not, and then the next time Mantyke is coming in at half health from switching into rocks twice.Water Absorb doesn't let Mantyke check very many offensive Water-types because they either carry the appropriate coverage move (Chinchou has Volt Switch, offensive Staryu has Thunderbolt, Shellder has Rock Blast), or they just muscle through it (Corphish, Wingull, physical Carvanha). The Ground resistance isn't very useful when Drilbur 2HKOes with Rock Slide after Stealth Rock (and avoids the OHKO from Scald), offensive Mudbray carries Rock Slide and usually at least ties with Mantyke, and Diglett doesn't give a damn if it's walled after it's already done its job. So what we're mostly left with for Mantyke to wall is a bunch of weak defensive Pokemon, like Ice Beam Staryu and Mareanie - but not the defensive Pokemon that are actually threatening, because those tend to carry setup moves, and defensive Mantyke is setup bait. In other words, Mantyke walls absolutely nothing of note. It comes in on barely anything, wastes its turns doing zero damage, and then it gets set up on by Ferroseed or Spritzee or something. Defensive Mantyke is useless, the most underwhelming set of all time. Berry Juice Lickitung is a more effective answer to big threats than Mantyke is; Baton Pass Skitty has a bigger niche than RestTalk Mantyke does. I hate defensive Mantyke.