Fuck, was Allen Iverson exactly 6'0"?
You just gotta follow the money. Unless they are from a foreign country, they are going to gravitate towards the highest salary, especially the supremely athletic as opportunities in that area will be more plentiful and promising than anything else in their lives.
So, it all depends on which type of athleticism. Insane fast twitch burst and durability, Football. Hands down. Running back or wide receiver at that. If it has to be under 6'0" it's a running back.
And it's assuredly Barry Sanders.
A balance between fast twitch reflex, balance, and cardiovascular endurance.... It's basketball. Under 6'0", Muggsy Bogues could fucking dunk and he was 5'3". That's like a 50" vertical leap.
But, yeah, Iverson deserves mention. The most athletically gifted basketball player ever. That is, under the assumption that frame is classified as something else.