Ali by a long shot but I have favorites in almost every weight class.
Watch the documentary When we were kings about the Ali /Forman fight in Africa for look into the genius of Ali.

Tyson was the opposite of Ali. Purely a fighter he was programmed by his handlers to be an assassin in the ring . If you watch his fights up to the Buster Douglas fight his opponents feared him, you can see it in their eyes and body language. It was only when Tyson lost that mental edge did he become beatable. They are all on you tube and fun to watch.

Watch the Boom boom Mancini , Duk Koo Kim fight to see one of the biggest hearts in all of sports history. Kim stood in the ring and fought like a tiger and it killed him.

Another fun one was the hands of stone Roberto Duran comeback against Iran Barkley. The 70's 80' were boxing a heydays
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Once you go black you never go back