Rocky Marciano was the greatest of all time. The only undefeated champion boxer of any time.
Plenty of undefeated fighters. Mayweather is also undefeated. So was Calzaghe, but no one calls him the greatest.
Most people blindly cite Ray Robinson as the greatest of all time, however it's obvious the overall skill level was far lower back then. Most of these people are also biased towards 20th century boxers. Rose tinted glasses. I've always found it amusing how in every sport where you can't accurately measure someone's skill, people just love to harp on about how much better fighters from the past were. If we had no way of timing Olympic sprinters, I guarantee, people would be going around preaching about how much better sprinters from 100 years ago were lol.