Jim Jeffries was, and probably still is the universal symbol of the masculine American tough guy. He was a product of the old world, and did everything that exemplifed pure testosterone, hunting, fishing, wrestling, manual labor, and of course fighting. A common reference is made to an incident where he carried a deer on his shoulders back to his camp after shooting it. Also, coming out of retirement after a significant layoff and taking on a champion who some felt was unbeatable at the time. I don't know if he was genuinely the toughest persay, and truthfully there's no way of ever finding out, but he certainly had the image and all the right criteria to be a top candidate.
Joe Jeannette was a real ironman in there. He salvaged a knockout victory from a 49 round beating delivered by fellow toughman Sam Mcvea. Jeannette mustered enough resolve in there to rise 27 times after being beat down mercilessly. Remember also that this was only with the 'colored' championship at stake, and was likely for mere peanuts compared to the purses of others.
Oh, man. Scrap that. I didn't realize we were talking about champions only. I wondered why Janitor didn't jump at the opportunity to plug Joe Jeannette. Gah, my powers of observation prove deficient once again. It took me a while to notice it too.
I actually think all (linear) world champions should be mentioned, with the exception of Sonny Liston.
Why exclude Sonny Liston? He might have disgaraced himself verry badly on one ocasion but on other days he showed considerable resolve in the face of adversity.
Rocky Marciano. His second fight with Ezzard Charles is a case in point. Nose split open like a walnut, and he comes storming out in the 8th, not running and hiding to protect that nose, but to swarm all over Charles to get a knockout. Just one case in point.
True, but no other champion ever quit as easily as he did. Therefore in my book, he cannot be ranked among them in terms of mental fortitude.
This bit about Marciano's legend is a little overblown. The fight was not in danger of being stopped, either by the referee or Marciano's corner. Weill's tale years later of him wanting to stop the fight was pure embellishment on Weill's part. As for the ref, well he said afterward that he had no intention of stopping it. No doubting Marciano's toughness though... My choices would be Ali, Jeffries, Willard, Frazier, Holyfield and Marciano.
In the case of that second bout with Charles, and that split in half nose, most past champions and most fighters, for that matter, may have had the courage to answer the bell for the next round, but would either be unable to handle the pain, and would have been taken off their game plan, and when the punches to that injury ensued, would have most likely not rallied with the deadly resolve that Marciano put forth. He was, as has been said, indifferent to pain, and virtually impossible to discourage.
He should probably be ranked above most of them in terms of "mental fortitude" but perhaps not as a boxer for the reason you state.