Luigi, because
A) I feel sorry for the guy, living in his brothers shadow.

B) If you pay close attention, Mario is actually evil. When Luigi wins in the tennis game, Mario grinds his boot into his foot. In Super Mario World, when you use Yoshi (my actual fave character)'s tounge attack, for a split second Mario actually hits him in the back of the head, and Yoshi flinches. Seriously, there's a video on youtube by the Game Theorists about it.