Roserade, Heracross, Rotom-H, Swampert, Flygon, Nidoking
This is a bit oversimplyfied. UU is quite a diverse metagame. While Flygon, Roserade and Heracross are indeed very common, there are other threats to consider when building a team:
1) Sand teams - Eviolite Hippopotas is very viable and sand is the only permanent weather in the tier, so Pokemon like Stoutland and many rock types that get the SpD boost are very viable and threatening;
2) Stall and quick stall teams are very viable as there are a lot of spikers such as Roserade, Deoxys, Froslass etc and a lot of spin blockers (ghosts) such as Mismagius, Sableye and the same Froslass;
3) Offensive boosters like Mew and Azelf should never be understimated as they can run through UU teams afrer a single NP;
4) Rain teams can be devastating now that snow warning is banned. Remember that UU has mons like Kingdra, Ludicolo and Kabutops;
5) Pokemon such as Empoleon, Krokodile are very common in top tier teams (at least according to my experience).
I've probably missed something but this should be a good starting point.