How do you even begin to approach this? Seriously...
Narrowing it down to the most athletic sport and then picking the best athlete in that sport? All around "athleticism"? Multi-sport athletes? How much someone can bench or squat, or how high they can jump?
That is such a subjective question... I would think there is some decathalete or triathelete, of whom I have never heard, who can dominate all other athletes in almost any series of measurable events. That would not make him or her particularly good at sports, but perhaps the world's best athlete.
