Guts? Who is it? Basically the guy that you could floor and knock around, but you'd never keep him down.
Gatti didn't have a weak, let alone terrible chin; the man was never a defensive marvel and simply ended up getting tagged flush throughout his career. Would you say, for example, that Danny Lopez had a weak chin?
seems inappropriate, but razor ruddock deserves a mention. he showed his recuperative ability vs tyson, lewis (two ATGs) not to mention tommy morrison, bonecrusher smith, etc. his chin was't great an you could drop him with grazing shots - but no one could put him down for the 10 - no one!
Carnera was Patterson before Patterson. As deadly a puncher as Max Baer was, Primo was getting to his feet almost as soon as he hit the deck. He was back on his feet almost as soon as Sharkey dropped him in their first meeting, but shrewdly dropped back to one knee for getting the benefit of a full count. Joe Louis couldn't keep him down either. In the cases of both Floyd and Preem, excellent conditioning was probably the key factor in their resiliency. I'd have to think a bit more about the non-heavyweights and non-champions, but Carnera's the most readily available example which hasn't been mentioned yet.
Just as CMC says - it's Floyd Patterson. But what about Dempsey v Firpo? Basically fighting on whilst semi-conscious or at the very least severely concussed? That shows astonishing fighting instincts more than bravery perhaps. There is a similair story about Louis in Schmeling I. I've never heard is second sourced though, so if anyone knows anything about this one, please post it.