By modern era i mean the late 70s to the present. The post ali era. Ali's last great fight was in 75 against frazier in manila but he wasn't that good after that and when sugar ray leonard won his first title in 79 it ushered in a new era when lighter weight fighters not just heavyweights got their due. The best fighter up to 1930 was probably jack johnson though willie pep gives him a run for the money. From 1930 to the 1960 it's probably sugar ray robinson but joe jouis, marciano, henry armstrong all deserve mention. Ali for the next 17-18 years. In the last 30 years there have been dozens of great fighters holding titles for a long time or winning multiple titles in several weight classes. It's not an easy call. How complete a fighter is and his ability to adapt to different styles was a factor and of course his record against other top fighters. I list 10 with 15 fighters in the honorable mention category. I'm sure someone will tell me how i'm wrong. We can also debate how certain fighters might have fared against each other. Whitaker vs duran, leonard vs trinidad etc. I have an idea how those fights would have turned out do you? It is how i decided on my list..