Ali was and will always be the greatest ! I also liked Roberto Duran, other than his no mas fight with Sugar Ray Leonard he was a great fighter over a long period of time.
I used to really like watching Arturo Gatti.
So many to choose from but one that hasnt been mentioned is Julio César Chávez his 90-fight unbeaten streak from the beginning of his career is the longest such run for a world-class fighter the sport has ever known. While his record was 89-0-1 going into his first loss to Frankie Randall and had an 87 fight win streak until his draw with Pernell Whitaker.