CoolStoryBrobat
The hero Smogon needs, but not the one it deserves
Now that I managed to sit down and get myself thinking, I've got a few mons to be nominated and discussed.
Clefable for B+ Rank given its Fairy typing, access to the extremely-broken Minimize, Calm Mind, and Stored Power, as well as recovery with Moonlight/Softboiled (good luck getting a legit one transferred from Fire Red/Leaf Green tho you hacker), it can be an incredibly annoying bulky setup sweeper for teams to hassle with, and has a fair number of setup opportunities given its typing. I haven't ran smack into too many of these but whenever I do I'm usually hard-pressed to kill it right away, especially if it's about to start boosting Evasion. Not to mention both its abilities are incredibly useful for it, Magic Guard making it immune to passive damage such as Toxic from Gliscor and Leech Seed from Ferrothorn, while Unaware lets it set up in the face of your own boosting sweeper with no consequence.
Conkeldurr for A- Rank because it is just so ridiculously bulky, durable, and threatening all at the same time. Assault Vest lets it sponge special blows left and right while it's able to keep itself going with recovery from Drain Punch. Guts discourages users from wanting to inflict it with status, while its movepool gives it all the tools it needs to threaten nearly everything between Drain Punch, Mach Punch, Knock Off, Ice Punch, and even Poison Jab, if you really hate Fairies that much. And if you wish to boost, Bulk Up is also an option, as is Choice Band for more power. All-around good Pokemon, and able to keep the majority of Dark-types, Steel-types, and status users (Scald users, Gliscor, Rotom-W, etc.) in check by itself.
Also cosigning Rotom-H for B+/A- for similar reasons to what SilentVerse said, but also because its typing is just incredible for the number of switch-in opportunities against various Pokemon...Also functions as a pretty good stop to Charizard Y among other things
Mega Manectric for either B+/A- as well. It's a great offensive support Pokemon and really great at grabbing momentum with Volt Switch while threatening most Pokemon with its coverage of Overheat/Flamethrower, Hidden Power Ice, and Thunderbolt. Blazing fast base 135 Speed lets it outrun even Mega Gengar while Intimidate lets it mess with mons like Scizor, Talonflame, Azumarill, Terrakion, Mega Pinsir, etc. It's often overlooked since it isn't a straight-up powerhouse (unless maybe you get a Lightning Rod boost before Mega Evolving) and it's easily worn down, but it checks so many offensive Pokemon by itself that it definitely deserves some credit.
Ferrothorn for A+. I can't believe we hadn't already ranked one of the absolute best checks to Mega Kangaskhan in the meta. Its combination of bulk + typing is insanely good, letting it get in on Azumarill, Excadrill, Garchomp, Venusaur, Gliscor, Gyarados, Landorus-T, Mamoswine, etc. It checks a huge range of the metagame, to the point that most of the mons it comes in on find themselves running a Fire- or Fighting-type move specifically to handle it. Rocky Helmet enables it to make Kangaskhan kill itself with recoil on top of Iron Barbs, though Lefties and even Occa Berry in some cases can work just as fine. Leech Seed sucks a ton of stuff dry, while Power Whip maintains offensive presence. And its typing is so easy to work with defensively, since its Fire- and Fighting-type weaknesses aren't hard to cover between 1-2 mons easily.
I got so much more, but lastly I'd like to just nominate Slowbro for B Rank. It's not conventionally seen and its typing and special bulk are kinda exploitable especially with Knock Off being buffed, but it's an absolute counter to Blaziken that still checks a bunch of physical attackers such as Dragonite, Gyarados, Excadrill, Garchomp, Landorus-T, Medicham (no one runs Thunder Punch anyways), Talonflame, Terrakion, Tyranitar, and a bunch of other guys. Also has Thunder Wave/Toxic with reliable recovery in Slack Off on top of Regenerator, so it's capable of supporting its team very easily
Clefable for B+ Rank given its Fairy typing, access to the extremely-broken Minimize, Calm Mind, and Stored Power, as well as recovery with Moonlight/Softboiled (good luck getting a legit one transferred from Fire Red/Leaf Green tho you hacker), it can be an incredibly annoying bulky setup sweeper for teams to hassle with, and has a fair number of setup opportunities given its typing. I haven't ran smack into too many of these but whenever I do I'm usually hard-pressed to kill it right away, especially if it's about to start boosting Evasion. Not to mention both its abilities are incredibly useful for it, Magic Guard making it immune to passive damage such as Toxic from Gliscor and Leech Seed from Ferrothorn, while Unaware lets it set up in the face of your own boosting sweeper with no consequence.
Conkeldurr for A- Rank because it is just so ridiculously bulky, durable, and threatening all at the same time. Assault Vest lets it sponge special blows left and right while it's able to keep itself going with recovery from Drain Punch. Guts discourages users from wanting to inflict it with status, while its movepool gives it all the tools it needs to threaten nearly everything between Drain Punch, Mach Punch, Knock Off, Ice Punch, and even Poison Jab, if you really hate Fairies that much. And if you wish to boost, Bulk Up is also an option, as is Choice Band for more power. All-around good Pokemon, and able to keep the majority of Dark-types, Steel-types, and status users (Scald users, Gliscor, Rotom-W, etc.) in check by itself.
Also cosigning Rotom-H for B+/A- for similar reasons to what SilentVerse said, but also because its typing is just incredible for the number of switch-in opportunities against various Pokemon...Also functions as a pretty good stop to Charizard Y among other things
Mega Manectric for either B+/A- as well. It's a great offensive support Pokemon and really great at grabbing momentum with Volt Switch while threatening most Pokemon with its coverage of Overheat/Flamethrower, Hidden Power Ice, and Thunderbolt. Blazing fast base 135 Speed lets it outrun even Mega Gengar while Intimidate lets it mess with mons like Scizor, Talonflame, Azumarill, Terrakion, Mega Pinsir, etc. It's often overlooked since it isn't a straight-up powerhouse (unless maybe you get a Lightning Rod boost before Mega Evolving) and it's easily worn down, but it checks so many offensive Pokemon by itself that it definitely deserves some credit.
Ferrothorn for A+. I can't believe we hadn't already ranked one of the absolute best checks to Mega Kangaskhan in the meta. Its combination of bulk + typing is insanely good, letting it get in on Azumarill, Excadrill, Garchomp, Venusaur, Gliscor, Gyarados, Landorus-T, Mamoswine, etc. It checks a huge range of the metagame, to the point that most of the mons it comes in on find themselves running a Fire- or Fighting-type move specifically to handle it. Rocky Helmet enables it to make Kangaskhan kill itself with recoil on top of Iron Barbs, though Lefties and even Occa Berry in some cases can work just as fine. Leech Seed sucks a ton of stuff dry, while Power Whip maintains offensive presence. And its typing is so easy to work with defensively, since its Fire- and Fighting-type weaknesses aren't hard to cover between 1-2 mons easily.
I got so much more, but lastly I'd like to just nominate Slowbro for B Rank. It's not conventionally seen and its typing and special bulk are kinda exploitable especially with Knock Off being buffed, but it's an absolute counter to Blaziken that still checks a bunch of physical attackers such as Dragonite, Gyarados, Excadrill, Garchomp, Landorus-T, Medicham (no one runs Thunder Punch anyways), Talonflame, Terrakion, Tyranitar, and a bunch of other guys. Also has Thunder Wave/Toxic with reliable recovery in Slack Off on top of Regenerator, so it's capable of supporting its team very easily