Depends on the game. N64: Pikachu because his move set was impressively strong. Melee: Mewtwo because it was just so nice having him in my game that I was playing at the time. Brawl: Meta Knight because he's [i]aggressively[/i] op. 3DS/Wii U: Bowser from the many buffs he has received, such as increased speed and a [i]much[/i] improved design from Melee and Brawl.