Jack Johnson, fighting before thousands of white men who would love to kill you needs more mental toughness than any fighter in history ever showed.
Examples of toughness that need to be mentioned: - Ali fighting with a fractured jaw in the first Norton fight. Ali lost but showed great courage and heart fighting with injury - Adamek vs Briggs I - Adamek broke his nose in sparing before his first title fight in his life but refused to postpond the bout. Adamek won close, bloody and brutal fight.
Oh it's gotta be Bad Chad Dawson after his excellent nay, BRILLIANT shellacking of the evergreen Milk Dud!
The three toughest guys I can think of...Rocky Marciano consistantly overcame facial lacerations , including that horrific sliced nose against Ezzard Charles, got up from a knockdown scored by the all time KO king Archie Moore, and was hopelessly behind on points and taking a beating against Walcott. Muhammad Ali fought with a broken jaw, but Arthur Abraham won his fight with a busted mandible that looked absoloutly gruesome.