Honestly IMO I think that Jack Dempsey would come out on top. Not only was Jack tough, but in one round of boxing, Dempsey broke Jess Willards nose, broke his jaw, shattered his cheek bones, broken his ribs, knocked out six teeth, and caused partail hearing loss in one ear. You cant tell me that Dempsey did not hit hard. And before straight out saying Mayweather, look up Dempsey's fights on youtube. Dempsey was one of the best fighters ever lived. A man with that much skill and determination would knock out Mayweather early in the 2nd round at the most.