Some people argue that JFK's actual body of work isn't that much compared to the fame he had and of course still has (I personally neither agree nor disagree with that one).

Ali came out of nowhere to take on a very good champion, never once thought he was going to lose; hell, he had everyone up in arms because he was so sure he would win, and he wins the fight and goes on to be someone who transcended his sport, his reputation and even his own persona.