Did anyone mention Herschel Walker? He played in the NFL, was part of the U.S. Olympic Bobsled team in 1992, almost made it as a sprinter. He also holds a 6th degree black belt in tae kwon do. (Source: Wikipedia)
So as far as greatest athlete he's got my vote. Hard to narrow this down though cause most people are looking at the greatest in just one sport, not across the board.