From fights i've actually watched (in no order):

Ali- Eventhough fnr3 giving him 50 out of a possible 100 (what a joke) i think he had a lot. He said no to the government for something he didn't believe in and served an unjust punishment. In Ali-Frazier I round 11, everyone thought he was rocked by the shot frazier hit him with but to me it looked like he was mocking frazier.
Litzau- Completely out on his feet but rallied and won the fight.
I'll say Marciano eventhough it's easy to keep fighting in a fight you're losing when you know all you need is one good shot and its over.
Chuvalo- Greatest chin ever or not you need to have something else to survive Ali, Foreman, and Frazier without being knocked down.
Chuvalo- Come on it can't be all chin can it?