In terms of dominance in their sport, diversity, stamina, accomplishment, and the X-Factor which cannot be measured on paper*. For those looking for Bruce Lee, I didn't include martial arts on this list.
*While Jackie Robinson may not have played better than Ty Cobb, he's higher because of the aforementioned X-Factor.
http://www.ranker.com/list/greatest-athletes-of-all-time/gamedemon24,
Babe Ruth
Bo Jackson
Deion Sanders
Magic Johnson
Hank Aaron
Joe Montana
Kelly Slater
Michael Jordan
Muhammad Ali
Tiger Woods