Any dragon type. I hate dragon type pokemon, because whenever I encounter them I have no pokemon that have the right type moves to bring them down.
anything other than Darkrai, even my new Arceus can't beat it -_-
I'm surprised nobody has said Wobbuffet. If one of those appears, you can't run, you can't switch out to a different Pokémon, thus, you're forced into a fight to the death if you ever see one, wild or trainer-owned.
As a few others have mentioned, fast Pokemon which hit hard and can cause major dents to your team. I generally always have some form of priority on my teams, specifically to counter any potential frail, fast(er) sweepers.
I'm surprised nobody has said Wobbuffet. If one of those appears, you can't run, you can't switch out to a different Pokémon, thus, you're forced into a fight to the death if you ever see one, wild or trainer-owned.