If doping doesn't disqualify you from this award, the Bolt is a good shout. Of all sports, sprinting probably has the most people attempt it, and Bolt is the best there's ever been. Not as big a sport, but if doping is allowed, then Lance Armstrong was also pretty good.not necessarily the answer to the original question, but Usain Bolt is my all time favourite
As for clean athletes - that's a tough one. But it's not Mohammed Ali. His mouth and status are bigger than his achievements.