Probably Mega-Gengar having the Shadow Tag ability. It's incredibly powerful anyway with a huge Special Attack and lots of different types moves, but it's pretty much guaranteed to KO something since you don't have the chance to get an appropriate counter up. And if you do, it's so fast and powerful that most of the time you have to hope that counter survives.
(Note that, at this point, I mostly play UU and not OU.)
So apparently, Gothitelle was banned from UU for similar reasons. I never really had a problem with, since I didn't encounter it that much, and I had U-Turn/Volt Switch/Roar/Whirlwind/Baton Pass/whatever on most of my Pokémon.
What really was OP in UU was Bisharp.
Boom! Knock Off! Bisharp has just rendered your precious Assault Vest/Leftovers/whatever useless! (which might work against it if it knocks off a Choice item)
Gonna attack? Nope! Sucker Punch.
Gonna switch? Nope! Pursuit. (in some Bisharp sets only)
Gonna send in a Fairy-type? Nope! Iron Head.
Gonna set up Sticky Web on a Bisharp team? Don't count on it. Defiant turns Sticky Web into a free Swords Dance.
Not to mention that a lot of things faster than it have paper-thin defenses, including Dugtrio. STAB Sucker Punch away! You'd think that Mienshao could be the answer to beating this thing, but it can only revenge-kill, since an Iron Head will OHKO it, and Knock Off and Sucker Punch both take over half of its health.
What's also frustrating is a Prankster Pokémon with Encore coming in on a setup sweeper and forcing you to switch out, since, barring ExtremeSpeed or being faster + having priority, there's no way you're not getting Encored again.