Phelps is a swimmer, not an athlete.
I would say the winner of the DECATHLON is the greatest athlete of the olympics. The Swimming medely is only 4 swimming strokes, whereas the Decathlete must be a sprinter, hurdler, jumper, middle distrance runner etc.
As a close second, or even equal with the decathlon, I would place the winner of the TRIATHLON, (swim, cycle, run) next, as this too, is a much bigger examination of an athletes all-round abilities than just running, just swimming etc.
Thirdly I would place the winner of the modern pentathlon.
Phelps and Bolt just excel at one aspect. Granted within the swimming field Phelps is clearly ahead of other swimmers in terms of exceptional versality, but it's still all swimming.
Bear in mind, 3 of his medals were also TEAM medals. He had the good fortune to be an excellent swimmer in a nation of excellent swimmers.
It is unfortunate thought, that the decathlete competes in 10 events and gets ONE medal, whereas a swimmer can get 8 medals... although, I guess perhaps you could say the decathlete could also go and compete in each individual event of the decathlon, and go for medals in those events too... hmmmm
How about this:
Greatest sportsman is: Gary Kasparov! It's all about the mind!!!