Wobbuffet Jr. (suggested by MGX):
Wynaut @ Oran Berry
Ability: Shadow Tag
EVs: 76 HP / 212 Def / 212 SDef
Impish Nature
-Counter
-Mirror Coat
-Encore
-Safeguard / Destiny Bond
Wynaut is the ultimate teammate for set up Pokemon and for counting Scarfed Pokemon. With it's very respectable HP and defenses, Wynaut can take a hit and use Encore, allowing you to Counter physical attacks, Mirror Coat special attacks, Safeguard vs status attacks, and / or switch out to a Pokemon who is barely touched by the attack being used by your opponent. This gives set up Pokemon a free turn to set up whatever they want, meaning it could get very dangerous really fast. Wynaut has always been forgotten because of the fact that it doesn't have great defensive typing (e.g., is weak to U-turn), is slow, and will likely get KOed before it does something. However, when Wynaut is placed on the right team, with the right pokemon, it will likely help another one of your Pokemon end the game, or come close to it on a regular basis.