To me a great athlete is one who changes the way his or her sport is played by dominating it in a measurable way.
1. The Babe. In one season he personally hit more dingers than any other team in the league. Not only was he the greatest hitter of his time he could also pitch. I doubt either of these feats will ever be equaled.
2. The Great One. Gretzky could almost score at will and he showed the sport that the assist is just as important as the goal. Probably the only person to change the game of hockey for the better in the last 40 years.
3. Tiger. Not much needs to be said here. We won't truly know how dominating he is for another 20 years but he's already matched or beaten pretty much every record in the book. The changes he has effected in the sport are already plainly evident.
If I were allowed a 4th pick it would go to Ali.
Edit: My three picks are numbered chronologically and not in order of greatness.