I honestly can't think of a scenario where Marciano beats Ali prime vs prime. Ali has pretty much every advantage you can think of, with his size and speed advantages being significant. Think about it: if Liston, Frazier, and Foreman couldn't stop Ali, how the hell could Marciano?
Prime Ali moved laterally very well, maybe best ever at heavy, and he could flick his jab regardless of which way he was moving. Rocky was an extremely tough man, but his cut prone skin would've NEVER held up to a prime Ali's jab and hook. Remember the mess he made of Henry Cooper?!?!?
The truth is that Marciano, along with Evander Holyfield, is one of the top two ATG overachievers in the history of the sport. The other truth is that he came along at the right time and was able to overcome his limitations to beat a cast of past their prime fighters such as Walcott, Charles, and Moore. Two of those guys were really only light heavies. Tough to see Rocky beating Liston, and Floyd Pattersons speed and skill advantages may have been enough to beat Rocky as well.
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