fantastic chin. He had mastered the peekaboo technique and the Dempsey roll and used it to perfection. Watch this sequence here:
https://www.reddit.com/r/fightporn/comments/e8tsxr/tyson/
Tyson in his later years didn't have the speed, athleticism, technique, etc... to pull off the phenomenal defense he showed when he was younger. I don't know if prime Tyson beats prime Ali, but it would certainly be close.