If phelps is there then more olympians should be there. People that argue about nobody knowing who Chris Hoy is due to nobody watching cycling, "who watches swimming" could be argued about Phelps!
Well Mayweather DID have a better 90's and so far 2010's......and by better, I mean, much better. Whereas Pacman was barely better in the 2000's, but due to Floyds 2 year absence, he may have it.
It's even harder to become a grandslam winner in tennis. So Andy Murray should be there? Top 10 being footballers would be as much a joke as this list actually is.
In terms of talent pool, no other sport (or even activity I'd wager) comes close. It'd be more than the entire population of North America - think there were an estimated 270 million playing football in 2006 and I suspect it's double now. To be in the top 20 of this pool is a ridiculous achievement. And then you have Messi who's on a different level - above and beyond these guys. It's staggering.