Greninja

Greninja has very high Speed, a great movepool, and the excellent ability Protean, which gives it STAB on every attack. However, it has pretty weak defensive stats and is thus easily KOed by common priority moves. It doesn't match up well against common Pokemon in the metagame, such as Mega Kangaskhan, which can KO it with the combination of Fake Out + Sucker Punch, and Choice Scarf Landorus-T, which can dispatch of it with U-turn or Superpower. Greninja's Special Attack is sufficient only when using super effective moves; its neutral attacks won't do much damage. It also loses pretty badly against speed control; Trick Room, Tailwind, and rain teams with Swift Swim sweepers destroy it pretty easily.