How about Willie pep? Not 100%, but I'm pretty sure he's the only boxer to have won a round without throwing a punch, just ducked, dived and slipped for 3mins and his opponent never landed a glove on him. That's pretty special. (if its true that is! Read it in a boxing magazine)
Would probably have to say Ali is the greatest though, not the hardest puncher but lightening fast and could also take a **** of a beating (thrilla in manilla, rumble in the jungle)