Has to be Usain Bolt 41 steps and 9.6 seconds to win every time in 100 meters. Greatness with far less work.
Federer all the way. Dan picks Michael Phelps? LOL! Far too myopically jingoistic of a move by Dan there. I love Phelps but he doesn't have the endorsements, nor notoriety on a world-wide level; nor will he be regarded world wide as one of the great athletes - and his sport is swimming which is only thought about for 2 weeks every 4 years. At least in Tennis it has popularity every year - and a much longer tradition. Just compare the notoriety of the likes of Connors, McEnroe, Agassi, and Sampras (even though Pete stays very private).
Federer is a much bigger athlete on a world level. On a personal level, you can't beat his life. And, he'll have more majors, surprisingly, than Tiger. I think about 5-6 years ago most would have argued for Tiger because he can compete well into his 40's.