For me, it's pretty much any unoriginal Pokemon that everyone just HAS to have on their team. Sticking to 6th gen, I have three in mind in particular.
Aegislash. Seriously, it gets a horrendously broken variant of the already-broken Protect move, and can leisurely hide behind it while it Swords Dances and Shadow Sneaks. It's such a pain to my physical powerhouses like Weavile and Flareon and is insanely easy to win with due to it's amazing coverage.
Talonflame. I usually don't have a problem beating this thing, but the fact that it gets a priority Brave Bird with almost no way around it really screams broken to me. I see this damn thing on literally almost every team I come across. Whatever happened to originality?
Mega-Kangaskhan. Not seen as often, and I usually don't say this, but you have zero skill as a trainer if you have to rely on this damn-near-uncounterable thing. Its priority 120-power Sucker Punch alone says something, but it also gets Fake Out, Power Up Punch, and Return to further break it. Takes the fun out of battling. Banning it was entirely justified.