Is no could have; is only is.

Without a standard it is impossible to say who was the greatest.

Cooper may well have beaten Ali except for Dundee and his knife.
The "LONG count" was what made Dempsey the winner in that fight.Firpo was a foreigner, Dempsey was an American. The ref was American.
Walcott beat Louis in the first fight but the judges robbed him. Louis even apologized to Walcott after wards.

The problem with boxing in those days was it was rigged, almost as bad as wrestling.