Rayquaza with Dragon Pulse, Flamethrower, Energy Ball, and Fly.
I used it to solo the Elite Four in Diamond around 200 times, probably, while I trained so many Pokemon up using Exp Shares to complete my Pokedex and for competitive play.
It pretty much had the perfect moveset to defeat everything the Elite Four had in that game, and would never take damage to anything besides priority attacks. Fly was just for flying back to the Pokemon League after I spawned back in my room.
Official Chespin of the Pokemon X boards. http://www.youtube.com/Decon082/