Hoo boy. This one's gonna be a bit of a comprehensive post to respond to TPP's prompt, but here we go:
I'm gonna have to go with
either Specs or Band for this one. The Choiced sets absolutely destroy Aegislash's decidedly limited counterplay and make it much, much harder to deal with.
Life Orb Clefable is the best wallbreaker in OU right now and this set has singlehandedly given Flareon and Centiskorch - two specially defensive Fire types - a niche worth noting. This set has virtually no defensive counterplay and offers so much defensive utility whilst being as strong a breaker as it is.
This one's difficult for me, honestly: I think its
Defog set (U-turn or Body Press+Brave Bird+Roost+Defog) is probably its best set because it provides teams with probably the only defensive countermeasure to Excadrill in the tier while also acting as one of the tier's very, very few viable Defoggers. A Steel/Flying type in a meta without Magnezone was destined to be amazing and Corviknight is that and so much more although its Bulk Up sets aren't quite as good as they were in the DMax meta.
Sub+Disable is, in my opinion,
by far Dragapult's best set in this current metagame. The tier has decidedly limited Ghost resists and the ones that do exist either have no business challenging Dragapult head-on (see: Hydreigon) or absolutely detest being burned by a stray Will-o-Wisp from this thing (see: the underrated Grimmsnarl). Sub+DD deserves an honorable mention for also being able to bluff Sub+Disable extremely well and is insanely difficult to revenge kill after a boost or two. Phantom Force is a shitty move but it actually doesn't suck
that much on this set either since it gives Dragapult extra Leftovers recovery and is insanely difficult to switch into after a boost.
Hatterene's
OTR set gives me Gen 5 Reuniclus flashbacks: if given a turn to set up it gives Offense an insanely hard time and it hits hard enough on its own to hurt Balance teams a great deal as well. Psychic/Fire coverage is phenomenal as it deals with FerroPex even if it isn't running Calm Mind. Being a fantastic Magic Bouncer helps a great deal, too; its very existence deters recklessly throwing around hazards or status moves.
I haven't used it enough in OU to formulate a truly educated opinion about it, but I think
Clangorous Soul+Throat Spray is one of the tier's nastiest sweepers if given a single free turn to set up. Kommo-o absolutely wrecks house if given an opportunity to get a Clangorous Soul off and the OU metagame isn't much better-equipped to defensively check a +2/+1 Kommo-o than UU is. It still has its defensive sets going for it too, bolstered further by having one of the strongest Body Presses in the game, so it has plenty of versatility backing it up.
I think
Scarf is still a pretty solid set despite Dynamax's ban; it provides a fast Volt Switch while maintaining good defensive utility and can provide teams with fast status in a pinch.
Rotom-Heat's best set, in my opinion, is probably a fatter
Nasty Plot set with Heavy-Duty Boots; Rotom-Wash probably does the Scarf set more justice. I don't really see all the hype for this mon besides acting as our tier's best defensive G-Darm check, and G-Darm probably doesn't have a very long future in OU.
BandTar may have lost Pursuit this gen, but I still think it hits hard enough with its enormous movepool and great STABs to be arguably its best set in this meta. Very little in this tier avoids the clean 2HKO if the TTar user predicts correctly.
As for the "underrated/not underrated" stuff...
Really good mon and a fantastic HO staple, but Corviknight gives it hell if it's running Body Press. I still think Bisharp is good. I think it's
particularly strong on HO teams running suicide lead Mew; they feel like DeoSharp, which I had a ton of fun using in Gen 6.
I think Cinderace is just okay in its pre-Libero state... before factoring in the ridiculousness that is Court Change+Heavy-Duty Boots. Court Change alone is enough to make this mon viable, and when it gets Libero eventually it'll easily be one of the best things in OU even before factoring in that busted-ass Court Change bullshit.
Dracovish doesn't have a 20% chance per turn of costing you the entire game. It hits like an absolute nuke 80% of the time but Bolt Beak is still a much worse move than Fishious Rend considering that Seismitoad eats it up all the same and there are a few other viable Ground types in this tier compared to the very few Water Absorb users we have. If Hustle didn't make everything Stone Edge levels of accuracy this thing would make Dracovish look pathetic by comparison. Overrated, IMO, but still good enough to have a place in OU.
Speaking of viable Ground types... Yeah, this thing is phenomenal. The degenerate at GF's studio that thought making Rapid Spin give +1 Speed would be a good idea deserves to be fired on the spot for this one. I think it's probably one of the best mons in OU right now - worse than G-Darm for sure and probably worse than Dragapult or Corviknight - but still one of the best. Corviknight is the only thing keeping this thing out of Ubers IMO.
Yeah, it's alright. I think Centiskorch is generally much better, but Flareon having a legitimate niche in OU is absolutely hilarious regardless.
This is the only true Dracovish counter. It is therefore pretty decent. It gives Rain teams a really hard time, too, which is nice.
Underrated. Nasty Plot makes this thing absolutely nightmarish to fight with a Stall team and Hypnosis gives it a surprising amount of free turns.
I made an extensive post about this thing a while back, but I'd just like to reiterate that I think Grimmsnarl is without a shadow of a doubt the most slept-on threat in OU right now and I think it's almost at the level of stuff like Excadrill. It has a fantastic defensive and offensive typing for the metagame and its utility movepool is downright massive; it is the best Dual Screens setter in the tier, it can function on balance with a defensive RestTalk set, it has excellent offensive sets that also offer plenty of utility, and in general it has so many potentially-viable options that it easily has the highest ceiling out of any mon in this tier right now. Please use Grimmsnarl more; it won't disappoint you.
Yeah, I don't think Jellicent is as good as I thought it would be at the very start of the generation. We have so many other Ghosts that shit on it offensively and a lot of the stuff it
wants to check defensively packs coverage to break past it (particularly Dracovish, which punishes its switchins with an absurdly strong Crunch).
Ice Shard to revenge kill Dragapult and Hydreigon is cute and Ice/Ground is still great offensively, but it can't break past a bunch of stuff nowadays and it's way too slow to do a huge number on most offensive teams without a bunch of support.
...what does this do, exactly? Or, rather, what does it do that Vaporeon or Toxapex cannot do better? I think Jellicent is better than Milotic and I think that mon is also pretty overrated. Is it used for its defensive attributes, or is there some weird offensive tech that's going over my head?
Don't let Hatterene being amazing distract you from the fact that this thing is still every bit as toxic to fight this gen as it was last gen. If you don't have a Dark type on your team Reuniclus will probably be your worst nightmare; unfortunately, Grimmsnarl utterly shits on it with most sets in my experience.
Lax has some potential but I haven't seen it used whatsoever on the ladder. Having a Ghost immunity in a tier with decidedly limited Ghost immunities is a huge deal and I think Lax therefore has a lot of room for experimentation.
Specs is overrated; Toxtricity's moves all have immunities and it can be a massive momentum sap if you predict poorly or if your opponent has an Excadrill in the back. Shift Gear+Throat Spray is massively underrated; it utterly fucks up a ton of teams, be they offensive or defensive, if given an opportunity to either set up or fire off a powerful move.
Is this used on Stall? I genuinely don't know what Xatu's role is currently, but I can see it being a downright kickass Magic Bouncer on fat teams in theory.
And, finally, my takes on the victims of a post-Dynamax meta:
It's definitely worse but I think it's still perfectly viable. I GREATLY recommend running Life Orb over Choice Band to gain the ability to switch up moves; it still packs some nasty coverage and is the platinum standard for speed tiers in this meta, with or without its Swift Swim boost.
Fuck this mon and fuck any meta where it comes even remotely close to reaching 40% usage. I hope it's relegated to being a niche pick since any meta in which Ditto is receiving 40% usage is unbelievably toxic.
Gyarados is the best Dragon Dancer still, albeit by a much smaller margin now than before. It can't wreck your entire team with Max Airstream into Max Geyser into Max Overgrowth anymore, but it's still a phenomenal setup sweeper that is still incredibly challenging to defensively check thanks to its newfound access to Power Whip.
It's still pretty decent, but it probably won't ever reach the peaks it reached in Gen 7 OU or in the Dynamax meta ever again. CC and Brave Bird are extremely useful additions to its movepool that give it a bit more self-reliance as a sweeper.
Shitmon.