... assuming that person has bad intentions.
My vote is Quentin "Rampage" Jackson.
There was a piece like this on the radio a while back, and the pick was Mike Tyson because while being able to kick your ass with a horrid punch, he's also fucking crazy, and would bite, spank, or rape your ass.
Well said on the Tyson comment. I'd pick Tyson simply because I wouldn't want to be with that guy in a church, much less an alley where he has bad intentions.
Rampage is no doubt a bad ass, but I don't think he is insane. You could probably talk him out of the beating he would give you. Plus, if you just fell and said you were sorry he'd probably stop the beating altogether, if it even came to that. He's got bigger fish to fry.
Tyson....not so much.