Douglas beating Tyson has to be up there. Ali's herculean effort in the first Frazier fight too. Foreman blasting Frazier out. An interesting one, but from a sheer guts and toughness perspective, I'd put Cobb against Holmes. Sure he lost every minute of the fight, but goddamn did he show a chin - and a nose - to eat all that leather.
Jersey Joe Walcott's win over Ezzard Charles, on his 5th try at the Lineal Heavyweight Title, has to be up there. That is some perseverance right there!
Jersey Joe Walcott against Joe Louis in their first fight Ali v Foreman Archie Moore against Marciano Mike Weaver against Larry Holmes in their first fight.
Usyk-Joshua I. Only a few of us knew he was going to do it. Dempsey-Willard Holmes-Norton. I'll give it to him. He was injured, and gutted it out. Liston-PAtterson is probably the right answer. Marciano-Walcott. Dealt with the adversity of getting the mysterious something in his eyes and swallowing a knockdown.