Several boxers were as famous as Muhammad Ali in the USA including; John L Sullivan, Jack Dempsey & Joe Louis [who was obviously loved more in his time than Ali in the 60=70's].
Remember; what is popular today may be forgotten tomorrow.
With that said; few people had Ali's global fame, i don't think even the great Joe Louis had Ali's fame.
The reason Ali was so famous is because of his controversial ideals.
Ali was the best heavyweight boxer of his time & he promoted himself very well, because of this he was famous in the Americas.
Ali, the most famous American athlete of his time, refused to fight the Vietkong & criticized USA racism, because of this he was famous in east Eurasia [unlike any American boxer EVER], in the dead middle of the cold war Ali had exhibitions in Moscow!; http://www.youtube.com/watch?v=06R4UVpkKtg
Muhammad Ali was also a muslim, thus [believe it or not] he was famous in the middle east & Africa [thus the crowd cheered for him against Foreman in Zaire].
Ali made major fights in the Phillipines, Africa & Europe while most all time greats either stayed in New York or in Las Vegas.
Only a few politicians have this kind of fame, but most are forgotten. In this last century men like Stalin, Hitler, Roosevelt & Mao may be as famous [but many people today don't know who they are yet they know who Ali is].
Obama is famous now but he will likely be forgotten by the general public like many previous presidents.
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Some say Michael Jordan surpassed Ali's fame, but Jordan was mostly famous in the Americas, the west & some parts of Africa. East Europeans, like Russians, know Ali, but Jordan never had that kind of fame in this region.
Muhammad Ali is a global figure thus he was chosen to light the Olympic torch.