Well obviously this information is outdated, but the most commonly seen Pokemon in competitive according to Smogon are;
OU: Scizor, Dragonite, Rotom-W, Tyranitar, Ferrothorn, Heatran Source
RU: Slowking, Druddigon, Entei, Spiritomb, Source
Anyway those are the Gen V ones I could find. There were extremely outdated ones for UU/Uber, but as of now my guess for in Ubers would probably be Kyogre, Arceus (Extremekiller :D), Palkia, Giratina, Darkai (I could be wrong :P just taking a guess). UU i'm really not sure :P They'd probably include Togekiss and Heracross.
Anyway yeah, OU is the most played tier