In a one on one fistfight, Tony. In a "political" fight, Michael. Tony is balls, rage, and rash judgement. Michael is reserved, calm, pensive, and methodical.
Tony was balls and rage but he at least had a heart - ignoring the abomination of Godfather III, you would swear Michael is the most cold and ruthless person in fiction...well, one that is a "hero" type. He'll hug you, tell you things are okay, then have you strangled or shot.
Tony wouldn't have killed Fredo so he wins in my book.