The LC council has finally gotten around to updating the Viability Ranking! Lots of changes here so bear with me.
The most obvious change is that we got rid of S- entirely. The time when Timburr, Abra, and Staryu were noticeably above the A+ Pokemon in viability has long passed; they are currently almost completely in line with other A+ and mid A Pokemon. Foongus is probably the most viable Pokemon in A+ right now, and it may have been a good fit in the old S-, but putting it there now would create the impression of a larger gap between it and the next best A+ Pokemon than what is actually the case.
S- -> A+
Timburr moving down is somewhat overdue; it's been some time since it could truly compete with Mienfoo in terms of splashability. While it is by no means less threatening or reliable of a Pokemon, and is as integral to the Fighting spam archetype as ever, the vast majority of teams will appreciate Mienfoo as their sole Fighting-type over Timburr. Of course, its strengths are more than enough for it to stand out on a large number of teams anyways.
S- -> A+
Life Orb Abra has seen a sharp decline in viability since USM; the introduction of Defog Snivy, coupled with the increase in Defog Wingull and Vullaby, means that webs is much more difficult to take advantage of, and its former top abuser certainly suffers from this. Sash Abra can now comfortably claim to be its most relevant set, but it doesn't enjoy facing specially defensive Berry Juice Vullaby or Beat Up Diglett. Various niche Abra variants such as Scarf and Modest have been seeing usage recently, but at the end of the day, Abra isn't nearly as splashable as it once was.
B+ -> A+
This one is probably going to be controversial, and there was definitely some disagreement on where Mareanie should place in A within the council, though all of us agreed that it needed to move up. For the time being, we have decided that Mareanie's absurd level of splashability and effectiveness is most in line with the weaker Pokemon in A+. While most Mareanie sets do pretty much the same thing, Mareanie is able to do its job quite consistently (even though this unfortunately does not include checking foodig). The diversity of what it can run in its final moveslot between Knock Off, Toxic Spikes, Iron Defense, and even Covet, coupled with a nice set of STAB moves, means that it can be fairly annoying to switch into repeatedly.
A+ -> A
Gastly hasn't really gotten worse; it's as unpredictable and immediately threatening as ever. However, A+ has gotten a bit better with S- moving down, and Gastly isn't quite splashable enough to remain there.
B+ -> A
With Mudbray and Chinchou becoming less and less common, most teams have their Volt Switch switchins limited to Diglett and Onix, which are far less resilient to Flash Cannon. Chinchou being less common also means that Magnemite can readily forgo HP Ground for HP Fire, HP Fighting, or even Sleep Talk. As a result, Scarf Magnemite's threat level has risen enough to almost approach where it was at in ORAS. Its typing is useful as always defensively, which is helpful for both Scarf Magnemite and less common variants.
S- -> A
Staryu simply isn't nearly as splashable as it once was. The ubiquity of Defog means that hazard-based strategies aren't as reliable, so its niche as the top Rapid Spinner is less noteworthy. 4 attack offensive sets with Eviolite or a z move have become much more popular, and are comparably threatening to Wingull while not being any harder to fit onto a team. On the other hand, defensive variants are in a very poor state, often being a liability for letting Pokemon like Foongus into play for free, and so only fit onto very specific builds, which isn't enough to place Staryu as a whole in a subtier above Wingull.
A -> A-
Spritzee, Doduo, and Mudbray haven't really gotten worse because of metagame trends, but while they aren't less splashable than Ponyta or Wingull, they lack the consistent effectiveness to warrant being ranked alongside the current mid A Pokemon. Similarly, Shellder is still a very threatening setup sweeper, but is noticeably harder to include on a team than the Pokemon in mid A due to offering almost nothing defensively.
A -> A-
On the other hand, Croagunk and Ferroseed definitely dislike a lot of current metagame trends. Diglett and Vullaby being everywhere, Wingull being the most popular frail bird, Mienfoo almost always running High Jump Kick to muscle through Croagunk, Timburr frequently running Fire Punch, and Gastly sometimes running Psychic all serve to make Croagunk's life difficult and ensure that it reliably checks almost nothing. Substitute variants let it take advantage of Foongus and Mareanie more easily, but it's still much harder to justify including on a team than before. Ferroseed dislikes Mienfoo running High Jump Kick and Timburr running Fire Punch to immediately OHKO or at least damage it beyond repair, and Magnemite and Gastly will often make room for HP Fire. On top of that, Defog makes it harder for Ferroseed to have a lasting impact on the game. The degree to which it loses momentum if it is to be preserved means that it fits poorly on teams that aren't either straight up balance or hyper offense.
B -> A-
Support Tirtouga has been on the rise over the past few months, and while this was initially for its reliability as a Torchic check, its strengths are just as noteworthy in the current metagame, especially with Nasty Plot Vullaby currently being one of its less viable sets. Being almost impossible to OHKO without a strong Grass-type attack allows Tirtouga to get up rocks even more reliably than Onix, and the threat of its Shell Smash set, coupled with its access to both Aqua Jet and Knock Off, allows it to force more switches than Onix to do so. Bulky Shell Smash sets have also gotten better for their ability to bluff support variants. Tirtouga definitely isn't perfect, as its support set is hard walled by common Fighting-types and its smash sets are as vulnerable to Scarfers as ever, but it can now seriously contend with Onix for the position of a Stealth Rock setting bird check on a team.
B+ -> A-
There are a few metagame trends that are allowing Carvanha back into the metagame as a legitimate threat worth building around. Croagunk and Ferroseed being less prominent is great for it, of course, but Mienfoo and Vullaby variants have also been getting somewhat frailer, with Vullaby almost always running Berry Juice nowadays. Other than that, Snivy is scarfed more often, Weak Armor is currently on the upswing, Staryu is more often forgoing Eviolite, fairies are less common, and some teams that only have one bulky Poison are choosing to run Mareanie instead of Foongus.
A- -> B+
Chinchou and Snubbull have been suggested to move down to B+ for a little while now. Now that all of A rank has been shifted down slightly, they are more clearly outmatched in terms of viability.
B -> B+
Vulpix-alola has a reputation for being a cheesy/gimmicky Pokemon, but at this point, such a reputation isn't really deserved. Well built veil teams are consistently able to take advantage of the move in all but the worst matchups, even if it's just from the threat of potentially setting them up should apix get into play safely; while apix lacking any defensive presence does make these teams a bit more matchup based than standard bulky offense, it isn't to a greater degree than, say, shelldig or carvanha-based teams. Furthermore, STAB Ice attacks are quite threatening in this metagame, which makes apix better in general and allows it to run variants other than Light Clay to some success.
B -> B+
Larvesta appreciates the ubiquity of Defog enormously. Its Scarf set has been seeing usage lately as it harder to force out with Wingull, Berry Juice Doduo, and other fast but frail Pokemon, while still having good revenge-killing potential if rocks can't be kept off the field.
B+ -> B
Wynaut, Kabuto, Corphish, Dwebble, and Rufflet aren't necessarily bad Pokemon, and can still be very effective on the right teams. However, they aren't splashable at all, nor do they have entire archetypes revolve around them the way higher ranked nichemons do. It's around the level of mid B that we see most of the impact of Torchic's ban. Kabuto's niche as a Rapid Spinner and Aqua Jet user is far less valuable, and it rarely finds more than one free turn anyways due to how easily it's worn down, meaning Tirtouga is usually better if you don't desperately need the role compression. Corphish's niche has been reduced to its offensive prowess with Torchic leaving the tier. Dwebble's hazard stacking set isn't nearly as effective because of how common Defog is, though its Shell Smash set may have gotten a bit better from frailer Mienfoo variants. Rufflet is on the verge of dropping further; most of the time, its immediate power simply isn't worth its lack of reliability, and its Bulk Up set is far less effective without Torchic support.
B- -> B
Trapinch's niche as a trapper is noticeably more prominent than most of the Pokemon in B- and even B. While its low Speed means that it'll far more often than Diglett have matchups where it isn't useful at all, and it is rarely able to trap more than one Pokemon, being able to directly switch into several of its targets such as Grimer-alola, trap Scarf users such as Pawniard and Magnemite, and immediate power all ensure that there are a fair number of teams where Trapinch is strictly the best choice.
B -> B-
Taillow is one of those Pokemon that's better on paper than in practice. In theory, special Taillow in particular is a great threat to a lot of teams with its good coverage and powerful STAB moves. However, it tends to lack the power to OHKO most Eviolite holders, often forcing it to rely on predicting perfectly to gain mileage, and many of the Pokemon that it does OHKO can force speed ties against it. Taillow is also quite easy to wear down between a Stealth Rock weakness, Life Orb recoil, and being frail enough for Mienfoo's Fake Out to 3HKO it after rocks. In conjunction with how difficult it is to get into play, Taillow can be quite challenging to take advantage of.
B -> B-
Munchlax is pretty bad in a Fighting dominated metagame, and it isn't immediately rewarding the way Steel-types are to make up for it. You really have to make the most out of its strengths for it to be worth a team slot.
C+ -> C
C+ -> C-
C+ -> unranked
C -> C-
C -> unranked
C- -> unranked
The other big change to the Viability Ranking is that we've finally fixed up the C ranks! For the longest time, C+ was pretty much the cutoff for Pokemon that were genuinely worth using on a competitive team, so it was really hard to judge whether something should be C, C-, or unranked. For some things, we might say "it's not worth using, but if you do use it, it can at least do this"; for others, we would say "oh it's not completely outclassed since it has a couple of things over Foongus, even though it's never enough to warrant a team slot"; and for others still, it would be that "it's really cute and I beat levi with it once. keep it". Low C being so messy deterred a lot of users from even touching it. Because of this, we've decided to lower the cutoff for actually being viable to the bottom of C-. If you think a Pokemon deserves to be ranked, you have to explain not just its strengths, but why these are enough to warrant a team slot over more viable Pokemon
even when considering the Pokemon's disadvantages. So while Bronzor, Venonat, and Drowzee may have made the cut in the old viability ranking, Bronzor is a flat out liability in multiple games for every game where it pops off, chainpass not being competitively viable leaves Venonat unviable as well, and in a horrific lapse of judgement, Hawkie shared the existence of Counter Drowzee with the world, causing the set to lose much of its effectiveness. Sorry!
C- -> C
C- was way too low by the old C standards (if you pretend they existed) for these two, and still too low now; they're at least usable.
B- -> C-
It's been literally years since Cranidos and Houndour were genuinely noteworthy offensive threats; in the current metagame, it's extremely diffficult to justify their inclusion on a team. I won't even bother listing all of the metagame trends that shut the two down, but Torchic's ban was the nail in the coffin for Houndour in particular. They can work, and their strengths are enough for there to exist teams that need them, but at that point, it starts to become a question of whether the team is really worth it when you have to resort to such poor Pokemon. This sums up most of the Pokemon in C- nicely.
C+ -> B-
Riolu was nominated a while back, and its effectiveness at gaining and maintaining momentum has proven to be a good fit for Voltturn-based Fighting spam teams to a degree that's more in line with Pokemon in B-.
C+ -> B-
B- -> C+
Amaura and Snover have switched places; Amaura is generally a lot more threatening if you're looking to include a strictly offensive Ice-type on your team, seeing how its Ice-type attacks are actually strong enough to 2HKO important threats. It also carries a handy Flying resist to get into play more easily. Snover still has its Grass typing, which may be helpful if you're particularly weak to Staryu, and Ice Shard gives LO variants a small niche on certain webs builds, but it's noticeably more difficult to take advantage of the effectiveness of Ice-type attacks with Scarf or Eviolite Snover when Blizzard doesn't even OHKO Foongus after rocks.
C+ -> B-
Finally, Meowth has been raised to B-.