I believe Rocky Marciano has a very good case after reading the following posts by ..

Carmelo Modica below:

My list of the top 5 heavyweights is:

1. Marciano, and in the next four positions, in no particular order: Dempsey, Louis, Tunney, Ali.

It isn't apparent at first, but when you start comparing Marciano's fights to any of the 70's boxers, you start to see what made Marciano almost unbeatable. He had virtually endless stamina, in fact he had the best stamina in boxing history. He punched for 15 rounds and never got tired. He would never give his opponent a second to catch his breath. Could you imagine if Ali had fought Marciano instead of Frazier? There would be no "cruising" through a round to recover energy. Ali would have been KO'd if he slowed down even for one second against Marciano. Watch the first Ezzard Charles-Rocky Marciano fight. No cruising allowed. Plus he was small enough to hit anyone on the chin. His bigger opponents were all brutalized. I would give George Foreman little chance against him. To beat Marciano, it would take a fast, smaller man like Gene Tunney, to outbox him and win a decision. But it would have to be Tunney on his best day ever. Marciano had better stamina than Dempsey. You would have to hit Rocky frequently to win points, but each time you hit, he would have a good chance to hit you with his stronger punches. Two guys tried to stick-and-move Marciano: Roland LaStarza and Harry Kid Matthews. They were both KO'd. LaStarza lost by SD the first time, but even there he was floored for 7. Marciano had lost a round for a low blow, or it would have been unanimous.

Every strategy has been tried against Marciano. Joe Walcott tried to give him everything he had in round one, in an attempt to knock him out early. Marciano was floored for 2 seconds and got up as if nothing had happened, and proceeded with his usual slow-but-steady demolition.

I think Tunney, LaStarza and Ali would have been capable of beating Marciano on points in maybe one out of five fights. But no more than that. Watch the LaStarza and Matthews fights on youtube and you'll see why I'm so pessimistic.