Well, one thing for sure. Marciano would crush Joe Frazier prime for prime... no disrespect to the great Smokin' Joe. But this doesn't necessarily translate to a win over Ali. Ali fought and defeated two much larger, and arguably harder punchers than the Rock: Sonny Liston, and George Foreman. And I agree with Mars from a few posts back. It depends on the version of Ali we're talking about. Coupled with Ali's speed, footwork, and boxing ability... you just cannot discount the physical advantages Ali also had (height and reach).