I see where people are debating whether Phelps is the number one athlete of all-time. It would be hard for me to put him ahead of Jordan, Pele and those that played team sports. He never had to worry about anyone impeding his progress. He didn't have to face a defense designed to shut him down. It is much easier for an individual sport athlete to dominate.
How dominant was he compared to other swimmers? I think by that measure he may well be the greatest. Jordan was great, but are Chamberlin, Russell, Bird, or Magic really that far behind? I don't think so. Not at all.
Kobe? That sorry fizzleer can't hold Michaels jock strap.
LeBron? Not so far behind. But he's still playing.
The point being, who is no. 2? Mark Spitz? A great swimmer, but Phelps is way, way ahead of that guy. I know training has improved, and the ability to support yourself while training, but that's true for all swimmers.
I think comparatively the case for Phelps can be made.