Sport: MMA
Bonafides From 2002-2012: Former UFC Welterweight and Lightweight Champion, 13-7-2 (4 KOs, 6 Submissions)
He's a psycho (this is what you call people who lick their opponents' blood off their gloves), but it's not like we watch MMA to indulge our rational instincts, so this dude is just about perfect for what he does. Along with Randy Couture, Penn is one of two MMA fighters to have held UFC 'chips in two different weight classes (Lightweight and Welterweight), and has been a participant in some of the biggest matches in UFC history. He announced his retirement after a loss to Nick Diaz last October, and while MMA retirements are far from iron clad, Penn has done more than enough to leave his mark on the sport.