Discovery Channel's Greatest American # 21: Muhammad Ali

Well, if your nickname is "The Greatest", and you're American, shouldn't you make the list by default?

Cassius Clay channeled his unmatched boxing prowess into a dominant position at the top of American culture in the 70s. How big was this guy? Well, DC Comics issued an oversized tabloid edition of a comic book in which he held his own with Superman. That's big.

When he refused to go to Vietnam, it was a turning point of sorts in the antiwar movement. Elvis went, Muhammad Ali didn't, and that was all you really needed to know about the erosion of public support for the war.

Ali was more than just an athlete--he was a cultural barometer, and a great American character.