I've got this post on Federer that has been growing for about a month. It's really long now and I suspect the quality is decreasing each time I add to it, luckily for you this means I won't post it. A lot of it revolves around the quality of Sampras and Federer's competition. Will you be surprised to read me claiming that it can be looked at in many different ways? No? Without boring you about why I settle on this measure, Federer wins because he has played a "much" higher percentage of his matches against fellow top ten players, 24% to Sampras' 16%.
I'm surprised that this is close to my intuitive impression. Clearly I am a genius.