So I know we have a slate but I think most of those have been touched on pretty heavily at this point so I have some nominations.
Victini B+ -> B
I think the reason it was B+ in the first place was the power of CB. Correct me if I'm wrong. Band isn't really that good any more with Tank Chomp everywhere, plus the defensive utility its typing offers (checking Zard Y, Mega Metagross which are less common threats now) has sorta fallen prey to the metagame trend of dark types on the rise. Scarftar is as popular as ever as well which is one of the best checks / revenge killers of Victini. The lure sets are cool and all but just on the premise of being rocks weak and not all the powerful I don't see them as enough to keep it where it is.
*corrects because wrong*
victini, i believe, rose to B+ because of its mixed sets. People love to orgasm over bulky chomp but mixed tini has a really clean way of dealing with it that you glossed over:
252+ SpA Expert Belt Victini Glaciate vs. 252 HP / 76 SpD Garchomp: 326-389 (77.6 - 92.6%) -- guaranteed 2HKO
And they have quite a bit of natural power too:
252+ SpA Victini Blue Flare vs. 252 HP / 84+ SpD Clefable: 177-208 (44.9 - 52.7%) -- guaranteed 3HKO after Leftovers recovery
coming close to 2hko'ing clefable without LO/band/specs/any plate is pretty damn impressive.
However, i do overall believe that it should drop because a large part of what makes victini good is still u-turn/v-create/bolt strike games and tank chomp+scarf tar still crap on those so it can stay B.
Mega Latias B+ -> B
So it did already drop because it lost its main niche, but building with this is more annoying than ever with the aforementioned rise of Dark types. Not to mention the qualities of being a Keldeo and Zard Y check aren't really as useful in this metagame where Keldeo is held a bit more in check and Zard Y is less common.
Empoleon B -> B-
I guess this is sort of a weird nomination because Empoleon has this undeniable niche in its typing + hazard control prowess. Problem being that trying to fit both SR + Defog onto one set applies too much pressure to do its job (whereas although it also pressures say Skarmory, at least it has Roost to mitigate this pressure somewhat). Granted, each individually is still nice but it doesn't really help when teams are already struggling to check electric types these days and without something like a Shuca / Chople berry it's vulnerable to a lot of coverage moves or common stuff like Sand Rush Excadrill. Running one of these just makes it easier to wear down and tougher for it to do its job. I don't really think it's as effective a defensive mon as Tangrowth or Gastrodon despite the hazard utility and typing.
More or less agree. Empoleon isn't as bad as you say it is but it should still move down.
Kyurem C+ -> C
Honestly never understood why this was ranked so high. I have been told what its niche is but it seems so damn specific that I find it hard to believe it's worthy of C+ rank. Is it really as good as things like Entei, Cobalion, Metagross and Seismitoad? I'd put it more on the level of C ranks honestly and could see it lower, but my lack of experience with it might have to do with that.
Yeah, it is.
This is mainly because of stall's inability to break it, rather than slowly chip at it. 404 hp subs+pressure outstalls chansey(and pretty much anything else) and ice beam+epower retains decent natural power against teams that aren't stall.
However, an often overlooked set that really gives it an edge over kyu-b is the specs set, which drops dracos that kyu-b could only dream of. It's as good as seismitoad/entei and certainly better than cobalion.
Some other I think should move down:
M-pert B+-->B tons of reasoning already shown for this
M-sharp B-->B- after messing around with it quite a bit, i can sufficiently say that it requires tons of support to get a sweep off correctly. Everybody's so prepared for bisharp and weavile that the list of shit you have to weaken just never ends. On top of that you want to run adamant to have some power but then want jolly because m-zam/aero still outpace you after +1. It's a huge struggle to get a proper sweep going and its really not worth the hassle, despite its super powerful crunch.
Starmie A- ---> B+ offensive sets last a few turns at best because they're so frail and defensive sets struggle to even keep rocks off the field vs tank chomp without ice beam, i find that both sets are pretty subpar and it's only saving grace is rapid spin. B+ seems like a better fit to me. There are plenty of checks even offense can use (kyu-b, azu, scarftar to name a few) and even balance can pressure it easily and stall can spinblock with msab. It can go down but B+ is a good fit for it.
Gallade B+-->B reasoning has been thrown around all over the place already.
Dragalge B+ --->B strong and tspikes can be cool but definitely not B+ worthy.
Dragonite B+ --> B so much fucking clefable
Anything below that i haven't seen/used much of to make a great opinion on
Already posted why cobal should move way way down tho lol.