Though some are controversial, I can agree on most candidates for Best Pokemon in UU. However, I'd like to step out from the train of thought people have to mention our well-known and beloved old fellow, Slowbro. One of its sets in particular, CMBro, is the one I've been using since my arrival at UU (I had tried it in OU at first, but felt uncomfortable trying to sweep with it in such a fast paced metagame, where potential threats lurk around every corner). Though I've had my ups and downs because, let's face it, I'm a mediocre player with no prior experience of the BW metagame (I'd played the first three generations, where Slowbro had a great niche as a bulky water but lacked reliable recovery to perform a sweep), I've managed to succeed to great effect with this guy, getting many sweeps, some of them cleaner than others.
Admittedly, I built my team in order to accomplish a Slowbro sweep so it hasn't surprised me much, but since I built the team with my own hands it was bound to have several issues with different Pokés, some of those I still can't get around (for example, Nidoking with Sludge Wave destroys me if
Snorlax isn't healthy enough, I'd imagine the same for Nidoqueen).
But really, Slowbro is incredibly powerful when used right, I've had no real issues with the most used threats since I have more than one answer to them (but for example, SD Heracross could sweep me if
Gligar has suffered prior damage and I mispredicted, letting it get an SD up) and after two CMs, it can withstand Thunderbolts from Offensive Zapdos or CM Raikou and KO them back, resists neutral hits from powerhouses like Nidoking like the best and has no problem beating the most resilient of walls, beating Porygon2, Umbreon, non-statused Milotic, Suicune... The best part is, it's not hard to get one CM up with the abundance of physical sweepers it stops short (heck, it isn't even 2HKOd by Scarf Krookodile's Crunch, Slacks Off Darmanitan's U-Turns and Arcanine's Wild Charges to force them out (for those LO variants)... The downside is, it can be revenged easily at +1, but it's nearly unstoppable late game, when I've taken care of those that could threaten it. And when it has a bad match up, like Flare Blitz said about Slowking, it can just switch out to recover a third of its max HP
The set I'm using has nothing abnormal about it, really, but just for the record:

Slowbro @ Leftovers
Trait: Regenerator
EVs: 248 HP / 244 Def / 16 SpA
IVs: 0 Att
Nature: Bold (+Def, -Att)
Moves:
ˇ Calm Mind
ˇ Scald
ˇ Psyshock
ˇ Slack Off
I can provide various logs of its success, most of them are long matches where I could mispredict or get haxed and still have a second chance (there's even a 168 turn battle that was really fun though very tiring). I'm by no means implying that Slowbro should be awarded the title of the best Pokémon, but I feel like it's not getting as much usage as it should, considering it got a special mention in The Smog's latest issue and I'd like to shed some more light on its prowess by nominating it among such renowned threats. I'm most experienced players know that Slowbro has amazing potential when used right, but this post in particular is trying to reflect that if a mediocre player like me can succeed not once, not twice, but tens of times in sweeping with Slowbro, it sure as hell means something, right?
Well, I won't bore you more with it. We all know that Heracross deserves that #1 (though admittedly, Slowbro can come in on a CC aimed at Snorlax from ScarfMoxieCross and immediately threaten the opponent with a CM sweep lol) and that Raikou and Kingdra are incredibly powerful and are solid candidates too (things like Roserade and Froslass get mentions because of their success at Spikes setting, though people should try using them differently; I can't tell you how much fun I have leading with Rotom-H when the opponent has Froslass on Team Preview, because everybody knows they'll lead with it; the same could be told for Mienshao, everyone Fakes Out and U-Turns, even though good players know Fake Out is a lousy move on the cat). But I feel like Slowbro should at least be considered, since I know it's not leaving UU anytime soon and I'd like to see it fighting the #1 spot with Hera, Rosey and the upcoming Mew)
For those of you that feel like me and spent some minutes reading my rant, here are some longass logs as a thank you:
I know those aren't awesome battles worthy of warstorying or anything like that (lol, my greatest battle so far had little participation from Slowbro, I may warstory it if I feel like it, because my opponent did great with some uncommon sets and the outcome was completely unexpected), but it just goes to show the potential Slowbro has, since it gets many opportunities to come in on all those physical sweepers it stops short. My team aids it a lot, which is good since that was the point of my teambuilding. I hope that, if you read them, you could appreciate the message I'm trying to send.
Sorry for the long post, I hope I did it justice and didn't come up as too one-sided