Phelps should get no votes.
He was at swimming, sure, but the only reason he gets so many medals is because (for some reason) swimming has different events for different strokes. I can't think of any other sport that does this.
A race is to get from point A to point B as quickly as possible. To arbitrarily add rules like "now you have to go backwards" or "now you have to flap your arms like a chicken" is ridiculous. Imagine if these were added to the 100m: not only would it be comical, but Usain Bolt would still win, and he'd be the one with 20 Olympic golds.
Don Bradman is one who stands head shoulders above all his peers. But is cricket a global enough sport?
I've always been impressed with multisport athletes, so would consider Bo Jackson and Jeff Wilson.
I've always thought that decathletes (and heptathletes) were the most impressive in athletics, and when I was younger Roman Sebrle was the man.