I say Roberto Duran, althogh I guess Ali could also be in the running, even though he was greater in the 1960's. Perhaps also Carlos Monzon.
Roberto Duran, he ruled the lightwieght division with a dominance .One loss ,avenged twice by ko.Left lightwieghts with 71-1 record.
Duran. Duran dominated the Lightweight Division like no other. The Uncle Tom sell out on your avatar had 20 Title Defenses in one of the weakest eras in the History of that Division.