I have to say it's Muhammad Ali. The guy fought one of the hardest hitters of that era in Sonny Liston WHILE HE WAS BLINDED.
Saad Muhammad Frazier Gatti Archie Moore LaMotta currently Amir Khan has to have a shout, Malignaggi's proven heart off the top of my head
A lot of heart when he fought, but when he was down he quit more than once. It happened most often toward the end of his career in rather hopeless fights, though.
Sugar Ray Robinson Muhammad Ali Joe Frazier Jake Lamotta I`ll just stop there because there are far too many to mention, those guys never failed in the heart department though, ever.
Muhammad Ali Joe Frazier Arturo Gatti Archie Moore Carl Froch Amir Khan Juan Manuel Marquez Evander Holyfield Are the standout ones off the top of my head.