Michael Jordon: is still widely considered to be the greatest basketball player in history (despite being an a-hole that the hall of fame induction)

LeBron James: the next Michael Jordon

Tiger Woods: Woods is only in his early thirties but only the Legendary Jack Nicolas has more tournement wins under his belt, a record Tiger will likely surpass in a few years, to become the undisputed greatest golfer ever.

Michael Schumacher: Still considered by many to be the best formula one driver ever.

Roger Federer: Mr. Tennis

Micheal Phelps: Only swordfish swim faster...

I thought I would throw in a funny one

Ric Flair "wrasslin" is not a real sport, but his accomplishments over nearly 40 years in mulitple federations is unequaled there will never be another.