Most people here mentioned athletes in sports that only interest Americans. No one cares about American football and baseball outside the U.S, and basketball is far less popular outside America than it is at home.
If you're going to have to name the greatest athtlete in the 20th Century, you'd either have to pic the greatest athlete in the World's most popular sport or the greatest winner of Olympic gold medals. In the first case, there'e a short answer to who's the World's greatest athlete in the 20th Century: Pele. As for Olympic gold medals, I think that the record is seven, and there are more than one athlete who's won that amount.
In any case, even when it comes to Olympic multi-medalists, it still doesen't tell as much, because they won several medals in only one sport, and they are usually sports like running, throwing and swimming that most people don't care about. In my opinion, the greatest athlete I can think of would be the guy who won the pentathlon the most times. There's a modality that combines five sports in one. I don't know who that was, but I do know for sure that Dolph Lundgren has won an Olympic gold medal in pentathlon.
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