Just a few comments as I think the rankings are in a pretty good state right now. Actually, C and C- are pretty bad, but that's pretty much just a glorified list of everybody's favourite nichemons so I'd rather not touch it with a ten foot pole. Good luck, Merritt.
Croagunk: A -> A-
Croagunk is one of those Pokemon that really thrives on being unprepared for, because as a standalone threat, it's exceptionally easy to build and play around. Croagunk has been up and down before; it was very popular during post-Misdreavus XY and early ORAS until Gothita/Acrofoo gained popularity, and was again bordering mid A around SPL 7 until offensive Vullaby/Flame Charge Ponyta started taking off. In the current metagame, Croagunk doesn't do a very good job at checking what it's supposed to. High Jump Kick does a good chunk if Mienfoo isn't carrying Acrobatics, Timburr sometimes forgoes Ice Punch for Fire Punch, Spritzee is carrying Psychic more often, Staryu runs Psychic more often than not, Snivy straight up muscles through Berry Juice Croagunk, and Grimer-Alola isn't as popular as before. Water spam is the only major archetype that Croagunk can check consistently, but it hasn't seen as much use at all this gen.
Of course, some (though not all) of those trends were already present in SM; the biggest hit to Croagunk's viability is that its own checks are so much more popular. A large part of Croagunk's power stemmed from its Nasty Plot set, which allowed Croagunk to act as a significant offensive threat that many teams left unaccounted for, especially if Fighting-types were lumped together while building. Nowadays, Vullaby, Onix, and Diglett are as huge as ever, while Wingull, Ponyta, and Mudbray are also large threats. Mareanie and Slowpoke, which can almost completely shut Croagunk down, are on the rise. With so many prominent checks on so many teams, NP Croagunk simply can't claim to be a large threat, forcing it to fall back on its status as a glue Pokemon to warrant a team slot to a much greater extent than before. And since it's only really solid enough to act as a secondary check to various threats, you would need multiple holes filled to justify Croagunk over a more threatening Pokemon. Croagunk will probably never drop below B+ in a metagame similar to ours just because at the end of the day, being able to check such a precise list of top threats is extremely valuable. But Pokemon in mid A tend to be consistently useful and/or threatening, able to pull their weight in almost every game, and between frequently not being able to do its job and not being able to do much else, Croagunk isn't the best choice for enough teams to warrant remaining in the subtier.
And yes, Diglett being indisputably top tier certainly hinders Croagunk's viability.
Mareanie: B+ -> A-/A
Foongus and Mareanie offer so much role compression in this metagame that I'm finding myself including one of the two on almost every team, and it doesn't look like I'm alone on this. Foongus may be more splashable overall because it doesn't stack a Diglett weakness and Spore is sort of busted, but Foongus's popularity also means that it's that much more prepared for. But the coverage moves for Foongus can't touch Mareanie Against teams without Diglett, Mareanie is almost always a godsend - a hard check to some of the biggest threats in the metagame that just doesn't die, coming in on Fighting-types, Torchic, most Spritzee variants, and some Pawniard variants over and over with little to no repercussion. Special emphasis on the lack of repercussion - several of the most prominents threats in the metagame aren't just checked by Mareanie, they're hard walled. Without Diglett, they have almost no hope of outlasting Mareanie's combination of Regenerator and Recover, turning into non-threats for the rest of the match outside of shifting momentum here and there. Mareanie is still quite passive once it's in play, but it's by no means ignorable - it can easily spread status with Sludge Bomb and Scald, and its newfound access to Knock Off doesn't hurt either.
Now, against teams with Diglett, Mareanie has to be a lot more careful - in addition to not being able to safely come in on Mienfoo anymore, it has to be wary of double switches against everything else. However, it's still not a dead weight since Diglett doesn't want to directly switch in, so coming in on things without VoltTurn is still an option since doubles are risky for the Diglett user too; it also acts as a safety net against a good number of sweepers. But really, its enormous presence against non-Diglett teams alone makes Mareanie worth it on so many teams that need the role compression and can pressure Diglett well; a significantly poorer matchup against a single common Pokemon is definitely a concern, but doesn't detract from Mareanie's strengths enough to prevent it from easily competing with the Pokemon in mid A in terms of splashability. If Mareanie rises in popularity enough for the things it checks to run coverage moves for it, then chances are Foongus will eclipse it and Mareanie will drop back down, but for the time being, Diglett is the only reason it isn't completely ridiculous.
Magnemite: B+ -> A-
Chinchou being on the decline is huge here. The net number of Volt Switch absorbers hasn't decreased at all, of course - Diglett and Onix are everywhere, and Mudbray is quite good as well - but none of them take Flash Cannon quite as well as Chinchou does. This makes switching into Magnemite directly quite risky and has aided the SturdyJuice set in particular, which can beat Berry Juice Mudbray / Onix if they come in on the Flash Cannon, and is tougher for Diglett to revenge-kill. Its niche as one of the more encompassing safety nets is quite valuable in a metagame that favours role compression, and it benefits from Foongus often running Synthesis, allowing Magnemite to hard wall it. It's particularly good on VoltTurn, which I feel is possibly the single strongest team type right now, acting as a slowish Volt Switch into Diglett or another VoltTurner, as well as a safety net for a lot of threats that could otherwise be quite annoying to the team, such as Shellder and birds.
ScarfMag doesn't benefit quite as much from Chinchou being less common since it always had more counterplay, and is still as annoyed by the other Volt Switch absorbers as ever, to the point where I'm not sure if it's outright more viable than SturdyJuice Magnemite anymore like it was in the past. Of course, it still has all the strengths that made it a top set back in ORAS; it's possibly the single most immediately threatening pivot out there with its Analytic boosted attacks, able to get a KO almost every time it comes in if it predicts correctly (and looking silly if it doesn't), turning it into a very high risk, high reward choice. It also works well on VoltTurn for its numerous resistances to get into play with.
Both sets are fairly splashable on general offense and offer a great deal for VoltTurn in particular, while also usually being able to play a large role in any given game. Magnemite may not be as consistently effective as most of the things in mid A, but it's far easier to include on a team than the Pokemon in B+; I think it would be a good fit in A-.
Cranidos: B- -> C+/C
First off, Choice Scarf / Eviolite support Cranidos variants that were seen in ORAS aren't very good. They were used to role compress Fletchling Pursuiting and Stealth Rock while still saving Pawniard for something else, but Fletchling no longer exists; Cranidos doesn't have the bulk to reliably check Doduo, loses to Wingull's Scald, and doesn't threaten Vullaby at all with Pursuit. With Fighting-types and Onix as common as they are, Cranidos has to predict perfectly to be able to threaten the opposing team, and even then, it's rarely able to OHKO an Eviolite-holding Pokemon and is easily forced out afterwards; in conjunction with its lack of opportunities to come into play, it's a high risk, low reward Pokemon that flat out isn't worth using. Speaking of Onix, it's a far better bird check since Pursuit isn't too useful on Cranidos.
The Life Orb set is still a decent webs abuser, but there are a few issues with this. Webs isn't nearly as good as it once was, so this isn't a particularly important niche to begin with. Furthermore, Cranidos faces a great deal of competition as a webs abuser - its vulnerability to common priority moves and the fact that it ties with many of the most important Choice Scarf users means that it's forced out far more easily than a webs sweeper should be, especially when you consider how webs teams tend not to have solid switch-ins for key threats, so it isn't consistent enough to be worth building the webs team around. It also lacks priority or a good speed tier, meaning it offers nothing for the biggest threats to webs in Snivy, Wingull, and Gastly, relegating it purely to being a dedicated abuser when it's not even a top choice for that. Cranidos's main advantage over other webs abusers is that it has the bulk to not be instantly revenge-killed by non-Wingull birds; however, this rather specific, and even if such an abuser is needed, Cranidos may not be the optimal choice, with plenty of other candidates that fit the bill and may offer something else on top of it. The fact that Cranidos has no relevant niche whatsoever puts it well below the Pokemon in B-, which are uncommon but potentially genuinely the best choice for certain teams, and even the Pokemon in C+, whose niches are extremely small but still noteworthy.
Snover: B- -> C+
Snover isn't quite as bad as Cranidos, as its unique typing and high base power STABs certainly allow it to be a fine choice on a few teams here and there, but its terrible stats hinder it from being consistently effective; for example, the standard Choice Scarf set loses to Foongus 1v1, with or without rocks. If you're looking to take advantage of a lack of Ice resists on most teams and nothing else, Amaura is generally the better choice, as it actually has the power to reliably 2HKO most Pokemon that are hit neutrally. Amaura is also the better Hail setter overall as it can also set rocks, although I'm not sure if this is relevant enough to be worth considering. When it comes to both roles, Snover can still distinguish itself by being able to take on Water-types in particular, as well as having access to (weak) priority, so it still has its niche; it's just much, much smaller and more specific than the niche of Cottonee or Salandit, which I would say are close to the middle of B-, and more in line with C+ Pokemon like Clamperl or Pumpkaboo-small in terms of how many teams genuinely have Snover as the best choice for the team.
Bulbasaur: C+ -> C
Bulbasaur is an acceptably viable Pokemon with an acceptable set of strengths that simply isn't able to find a place on a competitive team. The Celebrate set suffers from 4mss, only gets a single shot at sweeping, and generally lacks the power to break through a large portion of the metagame, making it difficult to justify building around; it's a fun set to use, and plenty of other mid C Pokemon are too, but there are far more reliable and threatening options available. You could use Bulbasaur to patch up holes with its decent typing, but access to Knock Off is nowhere near enough for Eviolite Bulbasaur to find a place on a team over Foongus, and z-Celebrate sets lack the initial bulk to check things well. Some Pokemon have a low ranking because they flat out aren't good, but Bulbasaur isn't one of them, so dropping it all the way to C- might be a bit much; it's just that other Pokemon are even better, more often.
Buneary: C+ -> C-
Why was this Pokemon ranked so high to begin with? It was never good to begin with, and as far as I know, nobody thinks otherwise. Z-Splash will never, ever sweep in a scenario where pretty much any decent sweeper couldn't, and non-Z sets are non-threats relative to any decent offensive Pokemon, with poor Buneary's poor offensive stats, lack of Knock Off, and lack of coverage on its STAB move. I'd never thought I'd say "just use Meowth", but if you're seriously considering Buneary for its offensive merits because it barely 2HKOes Timburr with LO Return, then here we are. On top of offering almost nothing from an offensive perspective, Buneary offers absolutely nothing from a defensive one. It does have one unique thing going for it: a fast Healing Wish. This isn't at all noteworthy, because Memento will usually do the trick, and there are Memento users (possibly with Z) that are better than Buneary in every other way.
Also, I disagree with the following:
Disagree - Torchic: A+ -> S-
Torchic may be on the verge of being banned, but if it does leave (which I'm not against), it likely won't be because Torchic is flat out broken. Strictly in terms of viability, Torchic isn't on the level of the S ranks. It's less splashable than everything in S, A+, and even some of A: it 1) has extremely defined weaknesses, which makes support like hazard removal and strong Water resistances mandatory; 2) opens up extremely defined holes in your team that need to be patched up, as numerous Pokemon are able to come in on the revenge-kill or Protect and punish Torchic heavily; and 3) acts as a reliable answer to literally nothing in the game due to its frailty and lack of immunities, meaning from a defensive standpoint, your team is one Pokemon down in being able to check major threats. Furthermore, while its strengths are certainly up there, they aren't the type where you can just throw Torchic on any team and expect it to succeed. Torchic needs a great deal of support to really shine, to the point where you'll want the entire team built around Torchic, as if you don't have the right Baton Pass recipient, you'll be stuck with a fast Pokemon with terrible coverage, which isn't worth it at all when you consider how much support Torchic needs to get into play in the first place.
And then there's the fact that Torchic-centric teams don't actually win more than other top strategies because they're quite unreliable in comparison - Torchic forces a ton of 50/50s between guessing the set, guessing Protect, and guessing the recipient, which are just as likely to go poorly for the Torchic user (outside of guessing the set, possibly) as they are for the opponent. Whether the sort of games Torchic creates are what we want is a different story, but in terms of sheer viability, Torchic fits the criteria for an S rank in neither splashability nor consistency.
Disagree - Snivy: A+ -> S-
I can see where this nomination is coming from. Snivy is arguably qualified when it comes to splashability; I would say that it ranks on par with Abra / Timburr / Pawniard, but below the S ranks / Staryu / Foongus / Onix. That being said, I feel that there is a difference in how consistently Snivy is able to pull its weight compared to the S ranks and Foongus, who I feel is bordering S- as well. The only common reason Mienfoo / Vullaby / Staryu / Timburr / Foongus would do nothing in a matchup is by getting hit by a crazy z-lure set early on; and by nature, Diglett / Abra will always have the potential to hinder the opponent in a significant way regardless of matchup, with exceptions arising from mispredictions or misplays rather than team composition. On the other hand, while it's quite rare, Snivy can become a non-threat against extremely offensive teams centered around sweepers that handle Snivy well, such as certain variants of birdspam and firespam; it also does very little against teams that simply pack enough Snivy checks and not enough Snivy-weak Pokemon. Of course, Snivy is very much worth including on teams because it'll at least be able to hold its own in most matchups, and as a sweeper, there will be some matchups where it absolutely dominates, but I feel that the fact that it can falter to such an extent against relevant archetypes is enough to exclude it from S.
Disagree - Slowpoke: B+ -> B
I imagine this nomination will have more merit once Torchic gets banned, but for now, I'd honestly say Slowpoke would do well in A-. Thanks to Regenerator, it's bar none the most resilient and reliable Torchic check in the metagame, and the closest you'll get to a hard stop to Torchic; it also has Thunder Wave and a good offensive movepool to ensure that Torchic's recipients don't come in for free. Its closest competition, Mareanie, risks being trapped by Diglett or Trapinch if Torchic predicts correctly. Slowpoke does struggle against Pursuit trappers somewhat, but it has the raw bulk to survive any Pursuit if it's healthy, especially if Scald burns as they come in, and threatens Pawniard in particular with Fire Blast. Torchic isn't the only Pokemon that Slowpoke checks - its huge bulk lets it avoid the OHKO from almost anything, allowing it to soft check a huge variety of offensive Pokemon that aren't Dark-type / Snivy / Spritzee, allowing for decent role compression. Slowpoke's downsides are as heavy as they have always been, but it has a very defined niche that is needed often enough to place Slowpoke in at least B+.
Ambivalent toward Houndour/Vulpix-alola dropping, but I'm not too fond of the reasoning presented for Houndour. It doesn't really compete with Torchic as Fire-type attacks aren't its only niche - just as important are STAB Pursuit, strong priority, and, well, checking Torchic. Pawniard and Ponyta are a bit more comparable, but Houndour remains different enough from them for teams that genuinely appreciate Houndour to not do well with having the slot replaced with Ponyta/Pawniard. That being said, Houndour just isn't very good right now, so I could see it dropping for that.
Re: Meowth
Get where you're coming from. However, your more recent quote says, "And yes, technician fake out + feint is decent, but that's why it's listed in the viability rankings due to not being exactly 100% outclassed by aipom."
This quote was taken out of context - several of the posts Gummy linked (Sken's included) simply go into why Meowth isn't a good Pokemon on its own. Meowth was never directly outclassed by Aipom, which is why it had a niche; the idea is that its niche simply isn't very noteworthy to begin with.
Now, a fair warning - many a great ladder hero has fallen in an attempt to defend Meowth's virtues here on the Viability Ranking. If, after reading through the relevant anti-Meowth posts that Gummy linked, you truly believe that you have something to bring to the table on the Meowth discussion, then I encourage you to first share your thoughts with a known anti-Meowth user and weed out the more obvious counterarguments before posting here.