Marciano, but it's closer than you think. Rocky is overated if you ask me he had 7 World Title fight's and his big win's were against past their prime Legend's like Louis, Moore, Charles and Walcott. I'd give Rocky some due that Louis, Moore and Charles are all top 10-15 ATG's but Rocky got them on the slide. Calzaghe fought a shot legend in RJJ and Hopkins looked all his 43 years in their fight despite rolling back the years against Pavlik. A lot of similarities with these two but Marciano has the edge for me.
Calzaghe vs Lopez would be more appropriate. Marciano and Mayweather got him beat by a country mile. Frankly, I think Lopez has a better resume as well.
Marciano has more impressive names on his resume, but if you look back at Joe C's resume, it has quite a few quality names than Rock. Eubank, Charles Brewer Robin Reid, RJJ, Hopkins, Kessler, Lacy, Bika, etc. Right now, I say Rock. 10-15 years from now, I might say Calzaghe.
I think its Marciano..not by a landslide though. Face it if this forum was active when Marciano was fighting.....Louis would be labeled as shot, Walcott would be labeled as shot, Charles would be labeled as shot....Moore would be labeled as past prime fighting at a higher weight. Calzaghes legacy will enhance by the year.....nothing any of these haters can do to change that.....it happens to all the greats
Though I love Marciano, I tend to think Calzaghe beats him on this. Calzaghe was the greatest Super Middleweight of all time, while Marciano was at best a top 10, no more than top three, Heavyweight of all time. Throw in Calzaghe's venture into 175 pounds and still going undefeated, that gives him all the more reason of being ahead, the success at different weight classes.
Marciano pounded on a punch drunk Joe Louis. He also beat a slew of bums/older fighters. Calzaghe's resume stands out.