George Foreman 5

Hit people so hard it was like he was using a crowbar on them. Demolished Frazier, who wasn't exactly a powderpuff, slamming him with body shots that hurt to watch.

Against Ali, he got schooled, his relatively basic techniques no avail against a smarter, quicker and better conditioned opponent. After that fight he seemed to re-invent himself, becoming a smarter quicker boxer.