Muhammad Ali or Rocky Marciano... me personally i know Marciano would have won.
I'm sorry to say that I disagree, but it would have been a terrific fight. Marciano was, to his credit, a phenomenal puncher, and could have punched a hole right through Ali's chest. . . ..IF Ali stood still, which he'd never do against a puncher like Rocky. Ali stood with harder punchers (Foreman and Shavers) and came out just fine. Also, Rocky was a really crude &boxer& (by that I mean, his boxing skills weren't exactly that of a Mayweather or Robinson). While I think Marciano could have been another Joe Frazier for Ali. . . .I think the Louisville Lip would have prevailed.
Both fighters have great chins! Neither were easily hurt. That being said this fight would be speed vs power. The thing I look at is the fact Ali withstood Foreman, Frazier, Shavers and Liston. I don't think Rocky at 188lbs hit any harder than Foreman or Shavers at 225lbs. I agree with blogbaba regarding Ali losing his best years. Unfortunately we can only speculate on how good he might have been. That being said Ali had a 13& reach advantage and about 18 lbs on Rocky, and speed never seen before or since. I don't believe Ali would Ko Rock, he was to strong. But Ali's speed and ability to take punishment and size advantage would make me believe he'd win on points. It would a great fight. A classic styles match up!
Neither were ever knocked out in the ring. Charles gave Marciano a boxing lesson for about 7 rounds only to tire out and have Marciano storm back and get the decision. Ali is quicker than Charles and certainly comparable in boxing ability. I think that what all this means is that this would be a very close fight and having seen Ali rest alot during rounds he would have to be in his best shape ever to be able to go the distance with Marciano. I have Marciano winning by a slim margin but in any return bout this could go either way.
Rocky was like a bulldozer. But Ali, could float like a butterfly But, I would have to give the nod to, Rocky.