First of all, I strongly agree with ranking Houndoom C+ at the very least, while its power may seem disappointing compared to Ninetales, it has more defensive utility thanks to its ability to come in on Ninetales with ease and also gets interesting setup opportunity vs stuff like choice locked Drapion, Gardevoir locked into Psyshock/Psychic thanks to its Dark-type. Also, its great coverage with its STABs alone gives it room for Taunt, which is huge since it actually destroys stall teams that rely on Pyukumuku to beat NP Ninetales, and it also prevents stuff like Mandibuzz and Registeel from potentially getting a Toxic off as you're trying to set up. Underrated mon, easy to overlook but definitely a threat.
I agree with HnC's nominations except for Shaymin (I'd sooner move Roserade to S than Shaymin to A+ tbh, both are very threatening and meta-defining Pokemon atm) and Aerodactyl (I'd have to see more of it in action to be convinced that it deserves more than B-, Ttrum seems like a better Rock-type in most instances). Also, I kinda wanna see more of Accelgor and Torterra cuz idk how they really work in practice.
As for my own noms:
A -> B: Rankings should always reflect not only how good a Pokemon is in and of itself, but also how good it is compared to other Pokemon, and for my money Bronzong faces too much competition from the superior Registeel to be worthy of the A ranks atm. It does have its specific niches over Registeel that make it worth considering: better matchup vs non-Knock Off Gligar, lack of Fighting weakness, the ability to be more offensively threatening with Gyro Ball, and the ability to deal with Nidoqueen and Flygon are nice benefits you don't get from Registeel. However, the last point is less relevant than it used to be, with the current meta highly favoring bulkier SR setters and Zydog over respectively Nidoqueen and Flygon. Meanwhile, some other meta trends (Stoise being so common, Drapion becoming a prominent trapper) make it harder for Bronzong to do its job and stay healthy. In addition, Registeel's extra bulk cannot be underestimated: 80/150/150 vs 67/116/116 is a seriously big difference and has serious consequences for each respective mon's longevity, and many breakers Registeel can check to a degree (Specs Yanmega, CB Ttrum, Golisopod etc) simply blow past by Bronzong by spamming their STAB move. So basically, the meta favors Bronzong's direct competitor in Registeel and as a result, the rankings should reflect Registeel's superiority.
A- -> B-: That this thing remained so highly ranked for so long kinda pisses me off, since it's a really awkward pick for most teams. Cress can be an alluring option, since its obnoxious bulk allows it to check a ton of threats at once, but unlike a lot of prominent walls, Cress offers very little outside of its ability to be fat. One obvious issue is that it can't do anything whatsoever to threaten Steel-types, and in this meta you'll want to run a Steel-type 9 out of 10 times because you need that kind of defensive utility on your team. Unlike other top tier walls, it cannot do much to actually threaten the opposing team: Milotic is fairly powerful and can spread burns, Gligar can Defog/set rocks and gain momentum with U-Turn, Registeel/Mega Lix/Bronzong can set rocks, Mandibuzz can Knock shit, but what can Cresselia really do? The aforementioned omnipresence of Steels makes it impossible to do much chip to your opponent's team, and Cress has no utility moves to speak of other than Lunar Dance. Since it's basically a dumb wall that provides very little for teams in a meta this offensive, I think it's fair to rank it alongside another stall staple in Pyukumuku, or at least knock it out of the A ranks.
B- -> C: Jellicent's niche is pretty minuscule, Milotic is a more reliable pick on the vast majority of teams and it happens to be just about the only true counter to (arguably) the number 1 threat in the meta (Mega Blastoise) and generally takes on everything it needs to wall a lot better than Jelli does. I guess what makes Jellicent appealing mostly boils down to its ability to beat non-Knock Off Toxicroak, which other Water-types can't do, and its ability to come in on Milo, although it doesn't do too much in return anyway. Being able to Taunt Registeel is nice I guess. Still, I don't think it's worth ranking this highly when it faces stiff competition as a bulky Water from Milotic and Mega Blastoise (as well as Slowking, Araquanid, Golisopod).
A- -> A: Speaking of which, Toxicroak has become a dominant force in the meta and I think the rankings ought to reflect that. Like most offensive threats in the A ranks, Toxicroak is very difficult to handle defensively thanks to its great coverage (which can be customized, with for example Knock Off providing you with a way to handle Doublade and beat Haze Milo more easily), but what sets it apart from something like Meloetta or Yanmega for me is its ability to come in several times and remain healthy thanks to its powerful Drain Punch and its unique typing + Dry Skin allowing for a decent amount of switchin opportunity. Having LO Sucker Punch is also incredibly useful in a meta that sorely lacks great priority users otherwise. The surge of some things like Ice Beam Milotic and Doublade make Toxicroak's life a bit more difficult, but to me this is just testament to how good it is in the meta and how easily it rips apart defensive cores when they do not properly account for the frog. Lastly I should mention that Scarf Croak has recently become more of a thing, further expanding its versatility. In this meta, offensive threats that provide more utility than "it breaks things" are extremely good, and Croak fits right into that category, so imo it should be up there with the best of them.
B -> C: KW nominated this down quite a while ago and I agree with him (not so much with a Mega Aboma drop, or at least not to B-), but I think B- is still too merciful. As a breaker, Mega Glalie tends to struggle too much with some of the biggest defensive threats in the tier such as Milotic, Registeel, and Doublade to be worth of your mega slot, so its main niche is that of an offensive Spiker. Four problems arise: first of all, although it gets Freeze-Dry it still kinda struggles with Mega Blastoise, the top hazard remover in the tier alongside Gligar. Secondly, it has huge competition from prime offensive Spiker Roserade, which does beat Blastoise and is a more threatening breaker to boot thanks to its excellent coverage options and access to Sleep Powder. Furthermore, it faces competition from two of the best Pokemon in the tier (Mega Blastoise, which would have been an excellent partner for Glalie otherwise, and Mega Steelix) by virtue of occupying the Mega slot. Finally, because it is a Spiker that is extremely weak to hazards, you're practically forced to run either Donphan, Forretress, or Xatu to keep hazards off the field while you try to set your own, and none of these are known to be amazing Pokemon. This is without even going into problems you'll struggle with in teambuilding afterwards due to the complete lack of defensive utility Glalie provides + the fact that you have to run a fairly mediocre Pokemon alongside it. My point being, Glalie is far too niche to be deserving of the B ranks, and I wouldn't even mind seeing it getting unranked at some point.
C+ -> D: Speaking of things that are not worth your time, this thing's ranking has been bugging me for a long time and I think it's time we did what's necessary. The fact of the matter is that Banette has no real niche that makes it worth using on a serious team and it takes up your Mega slot to boot. Defensively it's nothing worth writing home about, offensively it's too easily walled, its Prankster utility moves are a nuisance at best, as a spinblocker it obviously fails against Mega Blastoise, and in a meta where teambuilding is as difficult as it is you really don't have room for this thing.
C -> UR: The lower ranks could be deflated a little bit in general, this one for example really sucks since you're forgoing three amazing Grass-types for a mon that is too frail to defensively check the stuff you want it to like Zydog and too weak to dent anything, it's been in a few tour games as of late and in none of those games did it really do anything at all. Just really not worth using.
This post is getting too long so I'm gonna keep it short, but all of these mons could probably go unranked right now unless someone can prove they have a significant niche that actually makes them worth using over their competition: Cofagrigus, Smeargle, Tsareena (loses too hard to all common SR setters sorry), Hoopa (maybe not UR but could go lower cuz it's like impossible to fit on a team + Pursuit shits all over it), Galvantula, Delphox (maybe). Also, someone should nom Vaporeon, it's pretty decent tbh, definitely better than a lot of stuff in the C ranks.