Absolutely all of them.
Ali could have been, Robinson could have been, Duran could have, Armstrong, Greb, Charles....on and on.
For instance, Duran could have trained harder and not let his fat arse move him up like Toney today. He could have stayed at 140, beat Cervantes, moved up and beat Leonard, done better in the rematch and not quit, win or lose, beat Pryor later, not lost to guys like Laing etc. Could have trained as hard as he did for Leonard to fight Hagler and arguably would have beat him.
Anyone could have been better.