The greatest professional athlete in history?

[Reasonable]

Actually, since you mention 'in history' then I believe I'd argue many of Greek Olympians, such as Milo of Croton, were superior.

Phelps I just can't accept. He's an amazing swimmer, but competes in a sport where, under current rules, he gets multiple chances at gold, vs others who get one chance at gold.

If we're going to simply count gold medals then all events should allow all competitors equal amount of competes.

Also, I believe to be the best you must compete in multiple events vs overly specialize in one.

Boy can swim though!