I'm probably jumping the gun a bit here, but I'd like to make a rather controversial nomination of my own here:
Vileplume to C+ or B- (leaning towards C+ though)
Vileplume always had a niche in the tier and its current ranking of C reflects this quite well, but after the loss of Amoonguss to a spike in usage in
a much shittier tier OU this thing's niche has become far more pronounced and I really feel like, despite this recency, it warrants a rise.
First, let's get Vileplume's cons out of the way: it suffers from a fair bit of 4MSS, and it really wishes it could run all of Giga Drain, Sludge Wave, Gastro Acid, Strength Sap, Sleep Powder, Stun Spore, and HP Fire (to hit steels and particularly Scizor) but it can't. In addition, it lacks a lot of firepower since it needs to run a considerable amount of defensive investment to survive hits it wants to tank, despite having a higher Special Attack stat than its other Grass/Poison counterpart Venusaur. Because of its lacking offensive capabilities, Vileplume requires a fair bit of team support to truly shine: Magneton makes for a pretty solid partner if Vileplume isn't running HP Fire since it deals with the ubiquitous Scizor (though Strength Sap can ironically check Choice Banded variants from full since it outheals the damage from one hit and then proceeds to stall it out unless it crits). Nevertheless, I've often struggled to find the perfect support for Vileplume since it obviously isn't as self-sustainable as Amoonguss (and man do I wish this thing got Clear Smog) but I still feel that C+ or B- reflects this well, especially since Vileplume isn't strictly outclassed anymore. Lastly, a lot of the tier's premiere Special attackers, like Latias, go to town on Vileplume. Thus, Vileplume really appreciates some Pursuit support specifically to deal with Latias.
Now for the good stuff. For frame of reference, the set I run is this:
Vileplume @ Poisonium Z
Ability: Effect Spore
Nature: Bold (+Def -Atk) (IVs: 0 Atk)
EVs: 252 HP / 88 Def / 168 Speed
Move 1: Strength Sap
Move 2: Sludge Bomb
Move 3: Giga Drain
Move 4: Gastro Acid / Hidden Power Fire / Sleep Powder
Every single thing listed here has a purpose, though many aspects of this set may seem quite unorthodox at a glance:
- The Speed investment allows Vileplume to hit 178 Speed, allowing it to outspeed typical offensive Azumarill variants (and it can creep those that want to creep it at the cost of a little bit of bulk).
- Its HP is maxed out and the rest of its EVs are dumped into Defense so it can best check the things it's meant to check. It's still able to live a +2 Serperior's Breakneck Blitz from full about 43% of the time if push comes to shove.
- Vileplume's abilities aren't anything to write home about, but Effect Spore can occasionally come in really clutch by crippling something that makes contact with it (one of my replays showcases this quite nicely) and it's certainly better than Chlorophyll in a Droughtless meta. Being able to potentially Paralyze, Poison, or inflict Sleep on a contact move is way better than nothing.
- Strength Sap is the reason why you would even consider using Vileplume. It offers a very consistent and powerful heal and guarantees an Attack drop. When dealing with Breloom and Azumarill, this is crucial: both have very high Attack stats and Strength Sap consequently heals for a tremendous amount more, allowing Vileplume to repeatedly pressure them. If Breloom wishes to Swords Dance up, Vileplume can just Strength Sap and consequently put itself out of Breakneck Blitz OHKO range, forcing Breloom to either go for the full commitment and use the Z-move and get Sapped again, thus wasting its nuking potential, or risk going for a second Swords Dance, which can be swiftly punished by a Sludge Bomb. In Azumarill's case, all Choice Banded variants fail to OHKO Vileplume from full (and Ice Punch is a hell of a commitment to run regardless since you lose one of Liquidation/Knock Off/Aqua Jet/Play Rough) and get swiftly dispatched with a Sludge Bomb if weakened and an Acid Downpour if at full, or Vileplume can just go for the Strength Sap and force Azumarill to switch out instead. If at full HP, Vileplume has a chance to live a +6 Azumarill's Knock Off or Play Rough (it gets destroyed by Ice Punch if it opts to run Ice Punch though) and proceeds to either kill it or, again, stall it out with Strength Sap since it basically guarantees a full heal every time it's used. The -1 Attack it inflicts also allows it to not be a complete momentum sap (pun 100% intended) since an array of Physical attackers won't appreciate having to play around potential switches when they're doing two-thirds the normal damage.
- Sludge Bomb is Vileplume's strongest consistent STAB option and it hits Serperior, Azumarill, and Breloom for tremendous damage, and its Acid Downpour will OHKO all Azumarill and Serperior variants, including Sap Sipper+Whirlpool+Perish Song Azumarill variants.
- Giga Drain doesn't see quite as much use but it does considerable damage to Suicune, Hippowdon, Alomomola, Mega Bro, and the like.
- The last moveslot is a rather strange case: Sleep Powder, despite not being Spore levels of broken, allows you to cripple something willing to come in on Vileplume and deal with it with the rest of your team. Hidden Power Fire allows you to not be setup fodder for Scizor and does some damage to other Steels, although Scizor is its primary target and you may as well run this exclusively to hit Scizor since it's not really doing enough to other Steels regardless. Gastro Acid is definitely the most unorthodox option here, but removing certain abilities, like Poison Heal from Gliscor and the Breloom variants I never see, Technician from Scizor and the Breloom variants I always see, Huge Power from Azumarill, Tough Claws from Mega Aerodactyl, Regenerator from Alomomola, Natural Cure from Blissey, Filter from Mega Aggron, Contrary from Serperior, Pixillate from Mega Altaria, Strong Jaw from Mega Sharpedo, and the like, can really make this thing a pain to predict around since a lot of threats rely on their abilities to be threats. Do note, however, that I'm not at all implying that any of these can or should already come in on Vileplume, nor am I implying that Vileplume can or should come in on them and attempt to check them; this is just a general list of things that can be nailed by Gastro Acid, either as a prediction or as a parting gift so a sweeper can take advantage of a missing ability, since a lot of things really resent losing their abilities.
- The Z-move is also chosen over Black Sludge because it minimizes damage taken from Knock Off and cannot be Tricked off. Since Azumarill is one of the things Vileplume is meant to check it really appreciates taking less damage from Banded Knock Off, as does it appreciate having even the slightest chance of living a +6 Knock Off from full HP. Ironically, this was the first thing I thought about giving Vileplume when I decided to test it out.
Now for a handful of relevant calculations:
0 SpA Vileplume Acid Downpour (175 BP) vs. 252 HP / 4 SpD Azumarill: 492-578 (121.7 - 143%) -- guaranteed OHKO after Leftovers recovery (This is the Perish Song variant and is a guaranteed OHKO as is, so other variants get erased too.)
+6 252+ Atk Huge Power Azumarill Knock Off vs. 252 HP / 88+ Def Vileplume: 324-382 (91.5 - 107.9%) -- 50% chance to OHKO (It's a guaranteed OHKO after rocks though)
252+ Atk Choice Band Huge Power Azumarill Knock Off vs. 252 HP / 88+ Def Vileplume: 122-144 (34.4 - 40.6%) -- guaranteed 3HKO
0 SpA Vileplume Acid Downpour (175 BP) vs. 0 HP / 0 SpD Serperior: 426-504 (146.3 - 173.1%) -- guaranteed OHKO
+2 252 SpA Serperior Hidden Power Fire vs. 252 HP / 0 SpD Vileplume: 200-236 (56.4 - 66.6%) -- guaranteed 2HKO
+2 252 SpA Serperior Breakneck Blitz (200 BP) vs. 252 HP / 0 SpD Vileplume: 330-389 (93.2 - 109.8%) -- 56.3% chance to OHKO (Another OHKO after rocks, unfortunately, but still more useful as an emergency check if at full HP than nothing; nevertheless, Alolan Muk is a good partner for this thing because of Latias and it does a good job at keeping Serperior under control if need be too.)
252+ Atk Lansat Berry Breloom Natural Gift (100 BP Flying) vs. 252 HP / 88+ Def Vileplume: 226-268 (63.8 - 75.7%) -- guaranteed 2HKO (Substitute this for any applicable super effective Natural Gift variant as needed.)
252+ Atk Breloom Breakneck Blitz (200 BP) vs. 252 HP / 88+ Def Vileplume: 226-266 (63.8 - 75.1%) -- guaranteed 2HKO
0 SpA Vileplume Sludge Bomb vs. 0 HP / 0 SpD Breloom: 320-378 (122.6 - 144.8%) -- guaranteed OHKO
Some of these are admittedly kinda shaky, but Vileplume is functioning more as a good check than a good counter; I already explained why Breloom is generally at a disadvantage anyway, and most Azumarill variants have a hard time dealing with Vileplume anyway. Serperior is definitely the shakiest but it's still doable enough. And as mentioned earlier, Vileplume still has the tools to deal with other relevant threats in the UU metagame. It just struggles against other Special Attackers, but, again, it's not meant to check Latias, Hydreigon, and the like anyway.
Now that's all good and well in theory, but does it work in practice? Well, I'm certainly not the best UU player by any margin but I figured I'd test it out for myself. Here are two replays I had of Vileplume doing something pretty relevant, and I'm very confident that a more competent UU player can showcase Vileplume far better than I did both with predictions and with team support:
https://replay.pokemonshowdown.com/gen7uu-731998335
This was admittedly a pretty bad showing on my part very early on, and I lost Magneton very early to an Earth Power I should've seen coming from a mile away, but after a crucial Draco Meteor miss from my opponent I regained momentum and won from there. Though Specs Kyurem obviously did the most work by far, dispatching of Mega Manectric, Togekiss, Empoleon, and Arcanine pretty easily, Vileplume did exactly three things here that were absolutely crucial: it baited and ate Breloom's Spore, forced Breloom to switch out, and it proceeded to do huge damage to the Arcanine that pressured it into healing. Vileplume's ability to threaten Breloom quite easily can never be ignored. My opponent admittedly could've been better, but consistently putting pressure on Breloom was essential as it can all too easily stop Kyurem from doing some crazy damage by cutting its rampage short.
https://replay.pokemonshowdown.com/gen7uu-731814919
This, on the other hand, was Vileplume at its finest. It successfully managed to force my opponent's Breloom to switch out, it was able to Strength Sap the Defog Scizor, removing Serene Grace from Togekiss allowed me to far more easily pressure it with Scizor, and I was quite easily able to gain momentum from there on out. I opted to sac my Scizor to Stakataka (not a very smart option in hindsight, but it worked out) in order to trap and promptly eliminate it with Magneton, and Magneton proceeded to clean up quite nicely although I came pretty close to getting Flinch Haxed to death. After Magneton did its job I proceeded to use Vileplume as my wincon against the Breloom. The definitive MVP here was Effect Spore; deciding to simply get some high chip damage with U-turn ended up resulting in my opponent's complete downfall, and the Paralysis would've arguably been a better option. It's inconsistent, but Vileplume does a pretty good job at making U-turn spammers reconsider just freely throwing out the move.
I'll probably try to get a few more replays over the course of the next couple days so I can really put Vileplume to the test against stuff like Serperior and Azumarill (if anyone has any team options I'm all ears by the way), of course.
Anyway, to quickly summarize this enormous post/nomination: Vileplume requires a bit of support to function well, but with Amoonguss leaving the tier its niche as a defensive Serperior, Azumarill, and Breloom check is now far, far more pronounced.
Thus I would like to push for its rise to C+ or, potentially, higher.
EDIT: Taking the liberty of adding additional replays showcasing Vileplume every so often, starting with this:
https://replay.pokemonshowdown.com/gen7uu-732386726
This was a different account but I conveniently ran into the same person you may have seen in one of the previous replays. This time, though, I got a much better showcase of Vileplume both forcing Breloom out early on and giving me a lot of momentum by healing all the damage his Beedrill's U-turn did to it and giving his Stakataka -1 Attack, rendering Vileplume much harder to kill, and pestering his Togekiss on the switch with Gastro Acid although the more optimal play would've been an Acid Downpour (which would've been a hype but risky play; this showcases how much Vileplume ended up pestering his team. Though Specs Kyurem did a
lot of work this game, as it should, Vileplume functioned as an incredible wincon at the end, completely walling his Breloom set and easily managing to stay relatively healthy to check Beedrill. When the Beedrill finally came out, Vileplume absolutely went to town on it; though a crit would've obviously killed, its Poison Jab didn't do enough to kill Vileplume, and Strength Sap thus kept Vileplume out of even a crit's kill range. From there Beedrill was kinda forced to just attack into Vileplume as it outhealed it. It procced a Paralysis Effect Spore eventually, and I got Gastro Acid off to ultimately solidify my win against the Beedrill.
This was a VERY good showcase of Vileplume. It did everything it needed to but I didn't expect it to effectively wall Mega Beedrill like this on top of keeping Breloom away. I definitely wanna try exploring this thing a lot more!