Wobbuffet

At first glance, Wobbuffet appears to be a pathetic excuse for a Pokemon with awful stats, sans HP, and a movepool that's as shallow as can be. If one cares to look closer, however, Wobbuffet can be quite a threatening force. It possesses the ability to kill any Pokemon whom it wishes with proper prediction and the combination of Encore and either Mirror Coat or Counter. It can also use Encore to make a support Pokemon complete setup fodder. Although it's almost completely shut down by Taunt (Mirror Coat and Counter ignore Taunt), it is still definitely a force to prepare for; you can suddenly find yourself without a check to a very threatening sweeper or utterly helpless to do anything as one of the opponent's Pokemon gets a free turn of set up.