Wario. Not only he can pretty much knock-out other characters with a well-placed hit at low%, ignore most if not all projectiles (as he can eat them) and have a unique extra taunt no one else has, but he's the ideal character to stress out leeway players despite lacking ranged attacks (bike rush / charging fart), and the perfect companion for a Team Battle with Lucario, as you can be virtually untouchable (keep avoiding the adversaries with the bike) and grant automatically full Aura to Lucario, which in essence will kill the opponents pretty easily.
Also he's freaking funny to look at. I love crouching backwards at my adversaries (Wario basically starts taunting them with his butt). It's the extra disrespect you can grant them at the battle.
