For tennis, it's Federer. In his prime, he seems like he's not even trying while winning most of his Grand Slams with ease. Short of Nadal on clay, no one seemed close. Age seems to have caught up with him now. Looks like Sampras will keep his record of most total weeks ranked #1 in the world by 1 week. Federer still has the record for most consecutive weeks at #1 and most grand slam semis by a mile. He also has the most grand slam titles as well. I didn't really was into Tennis until about 10 years ago. I'm glad I got into tennis in time to see Federer through his prime.