This is something I would really like to see develop into something more.
After having tested some things here and there with some other Showdown users, I have kinda gotten to see what is good and what isn't, and this is some sort of an overview of the top threats(imo).
Nidoqueen is probably the best mon, simple as that. Few things arent 3HKOed, and those who aren't can simply be put to sleep. Nidoqueen also serves as a nice check to non-HP Ground Victreebel, which is incredibly good.
Slowbro is amazing too, with good offensive coverage, and great defenses and T-Wave, its definitely up there with Nidoqueen.
Quagsire would probably have been THE best if it wasn't for the fact that everything runs HP Grass just to check it. I still find him to be one of the best sponges in the tier though, walling the threat called Electabuzz(without HP Grass since it's uncommon apparently) to hell and back, while his Curse set is probably one of, if not the best sweepers in the tier.
Chansey is of course going to be the premier special wall, and nothing probably goes past her that way, so stop trying. However, being unable to have Heal Bell and Seismic Toss on the same set(at least it's impossible on PO), kinda limits her on the offensive, and can get set-up on easier because of this.
As for the things Nidoqueen doesn't straight up slaughter, we have Slowking. Slowking is usually overshadowed by Slowbro because of worse stat distribution, but in this case, it's for the better. Unlike Slowbro, Slowking is NEVER 3HKOed by Nidoqueens Thunder, where as Slowbro is always 3HKOed. With Slowbro running T-Wave over Sleep Talk, this turns Nidoqueen into an even more dangerous foe.
We also have the sorta gimmicky, but perfectly viable Reflect + Ice Beam Chansey, which easily stalls out Nidoqueen as long as it avoids crits(EQ does ~40%). You forgo Toxic, which can be game-changing in a stall-game, but offense is generally better(and more common), so you wont really loose out on too much.
Nido can also do little to Quag aside from sleeping it. This makes RestTalk Quag the best Nidoqueen counter in the tier, as it can Curse up on it as it is forced to switch(watch out for flying-types, as RestTalk means no HP Rock, unless you forgo Curse), or just EQ it to death.
Hypno is good too, as it functions as a worse Quag, taking hits from Nidoqueen and Electabuzz fairly well, taking ~35% from Nido's EQ and 30-35% from Electabuzz's Thunder. It can also support the team with Reflect and Thunder Wave, or Sleep Talk if it so desires.
As for something I had high expectations for, but hasn't really lived up to the hype, is Ampharos. While being a good supporter and all, it's electric typing isn't as good in UU as in OU. Giving almost free switches to Nidoqueen or Quagsire(depending on HP) is something you rarely want to do, and therefore I feel it kinda falls between the rocks. It also dislikes the faster meta UU has to offer.
Other good Pokemon that deserves a mention:
Dodrio, Victreebel(mega-huge threat, just haven't played with or around it enough yet), Gyarados, Kabutops, Vileplume, Magneton.
Things I just haven't faced enough to really give an honest opinion on:
Arcanine, Politoed, Poliwrath, Magmar, Scyther, etc.
Really hoping Smogon can write up some UU analyses, and post them to get this meta going, since I find it interresting and fun to play.
EDIT: Forgot to mention Lanturn, really nice mon against Slowbro, and other waters not named Quagsire. Sponges most special hits easily(unless it's STAB Thunder, that hurts).