Out of my own curiosity, I had to check some stats among these 3.
Sampras
101-20, .835 overall, 1.37 D.R.
11-3, .786 vs top-10, 1.20 D.R.
6-0, 1.000 vs top-5, 1.49 D.R.
Federer
192-29, .869 overall, 1.38 D.R.
21-11, .656 vs top-10, 1.15 D.R.
11-6, .647 vs top-5, 1.11 D.R.
Djokovic
120-20, .857 overall, 1.33 D.R.
16-10, .615 vs top-10, 1.08 D.R.
6-7, .462 vs top-5, 0.95 D.R.
other notable stats:
longest winning streak on grass:
Federer 65
Djokovic 34
Sampras 23
longest winning streak vs top-10 on grass
Federer 11
Sampras 8
Djokovic 6
I have to go Federer > Sampras > Djokovic here.