Hmmm...I think I'll pick:
Gordon Freeman - A scientist that has no military training whatsoever and is able to repel Spec Ops and a invading alien force, all without uttering a word? Bring him in!
Captain John "Soap" MacTavish - Here's a man that was thrown from a helicopter, was shot who knows how many times, was blown up by an oil tanker and still isn't dead, but instead is excelling in the field of bad-assery. Oh yeah, he's a member of the 22nd S.A.S.
Altaïr Ibn La-Ahad - Come on, he's the most bad-ass assassin ever! He wears all white and you still don't see him coming! And don't forget the Leap of Faith (jumping from the tallest structure, the steel chapel in Acre all the way down to a tiny hay cart down below, there is few moments that can beat that in terms of bad-assery)
The Secret Apprentice - Have you seen him in action? It's wicked crazy! Wild force powers that blow away whole rooms of people if he so much as coughs, skills with a lightsaber that would make Vader blush all rounds up to one deadly bad-ass user of the force.