The Ultimate Man of Manliness is Duke Nukem. This is a fact, not an opinion.
I also feel that old heroes like The Nameless Space Marine and BJ Blakowicz deserve a mention.
A man who tangents the Awesomeness of Duke is Kane. Completely insane, yes, but awesome none-the-less.
Master Chief does not win. He looks much like a Nameless Space Marine clone, and so all awesomeness he has is distributed to him. Gordon Freeman is a good contestant, though.