Talk about a can of worms question.
How do you define 'the best'?
Number of medals....Phelps, clearly.
Longevity? Sir Steven Redgrave maybe. 5 consecutive Olympics with gold in 5 consecutive Olympics.
Entertainment and overall awesomeness? Bolt....but unfortunately for me there will always be a question mark on if he is clean (and I REALLY hope he is)
Freakish ability? Emil Zatopek. Won gold in the 5000 and then the 10,000m at Helsinki and then thought what the hell, I'll enter the marathon....yep, won that too. A feat never again replicated nor possibly ever will be.
Nadia Comenici....the first perfect 10 in Gymnastics. They had to change the scoring because of her. Now it is not possible to attain a perfect score.
Muhammad Ali....well, he was the self proclaimed 'THE GREATEST', and don't forget, he was an Olympian.
I don't think you could ever come up with 'the best' Olympian ever. Too many variables. Too many events with stand out athletes that in their own rights have achieved 'the best' status.
If I made the Olympics I would have said I was the best