if he didn't have a great chin he would have been oft stopped.
he gets tons of credit for it in hardcore boxing circles, but outside of them it's never mentioned. ali had a great chin.
he wasn't actually as hard to hit as people make it out, especially after his layoff. he had great range and physical and mental speed, but he had some fundamental flaws in his defense. he used to pull straight back from punches. henry cooper, for instance, hit him with the left hook.
ken norton broke his jaw
frazier beat the snot out of him in the two fights they had that were close. they beat the snot out of each other, to be fair.