The Real Deal. "If he weighs 205lb then his heart must weigh 204lb" - Larry Merchant on Evander Holyfield
It's a dumb question. Why is it we all must assume that the guys who come forward constantly and take punches are by definition having the most heart? How does one even define this? Maybe a guy went into a fight with an injury and fought anyway. Maybe some tragedy befell him before having to go into the ring against some tiger, and he had to find a way somehow to focus despite all that inner turmoil and box smartly with a smartness and discipline few could show under such circumstances. Hell, just getting into the ring is a show of courage in itself. There's no way to quantify any answer.
It's damn near impossible to measure heart when fighters reach a certain level of what they show. Gatti, Corrales, Chacon, Frazier, Saad, etc... Overused cliches in tough fights are "it's who wants it more," or "it's about heart at this point"....how would you know that, for example, Ali wanted it more than Frazier in Manila? Could it be that Ali was a better fighter and had a little more in the tank? What makes Gatti have more heart than Corrales, or vice versa?
Joe Frazier and Rocky Marciano spring to mind. There was no quit in them at their peaks, and in Marciano's case, never ever.
I find myself agreeing with this. Even in a single fighter such as Holyfield who used to war and brawl with huge men, used his diminished capacity in old age to fight in a considerably different style. I don't think he had any less heart at that point, though.
In answer to the original question I'd say Don Cockell but I am kind of of the mindset of salsanchez here - Billy Miske is another guy you would have to say from that angle as well
Hagler, Frazier, Holyfield, and out of today's current fighters, Miguell Cotto seems a worthy mention.