Ali, in his prime, would've boxed rings around Marciano, I think.

The level of competition in the Heavyweight Division was much stronger during Ali's day, and he fought and beat just about everyone during his 17-18 years on top.

While Marciano was undefeated, he was not on top very long before retiring, and didn't beat nearly as many "name" fighters in their prime as Ali did.

The four best that Rocky faced were Joe Louis, Ezzard Charles, Joe Walcott, and Archie Moore, all of whom had seen better days to some degree.

The guys Ali fought and beat were mostly in their prime when he beat them.