Of course athletic ability would be the prime determining factor for any list but I'd add "heart" to my list as a close second. Heart, the urge to succeed in your field, the intestinal fortitude to go on when you're on the ropes, the desire to give your all.

I try to discount the current or near-current "stars" when compiling such an all-time list. There's a lot I weigh against long time performance by a particular athlete.

I also add some credence to public appeal.

With all this in mind I'll name some that come to mind right now (in no particular order):

1. Jim Thorpe - probably the greatest athlete of the 20th century. He won the pentathlon AND decathlon in the Olympics, played professional football, baseball and basketball.

2. Babe Ruth - probably the single biggest name in sports of the 20th century. His prowess with the bat overshadowed his wonderful pitching skills, too.

3. Muhammad Ali - arguably the greatest prize fighter of all-time. His speed and boxing skills overshadowed decent power and the ability to take a punch.

4. Secretariat - IMO the greatest racehorse of all-time. Winning the Belmont Stakes by 31 lengths is one of the greatest single best athletic performances ever. He defined "heart".

5. Tiger Woods - awesome physical skills, concentration and desire add up to make him the best of the best golfers.