Well, whom did he beat? Taking it from 4th round on at Wimbledon, we have:
1993: Andrew Foster (British wild card), Andre Agassi (defending champ), Boris Becker (past peak but still good), Jim Courier (at peak though hardly a grass-courter).
1994: Daniel Vacek (who?), Michael Chang (decent player, not a grass-courter), Todd Martin (good but not great), Goran Ivanisevic (good on grass).
1995: Greg Rusedski (ranked 60th at the time), Shuzo Matsuoka (one for the connoisseurs, shall we say?), Goran Ivanisevic, Boris Becker (past peak, still would win another slam next year though).
1996 he lost to Krajicek in the quarters.
1997: Petr Korda (decent player, hardly a demon on grass), Boris Becker (past it by now), Todd Woodbridge (doubles specialist), Cedric Pioline (decent, far from great).
1998: Sebastien Grosjean (ranked outside Top 100 at the time), Mark Philippoussis (good, not great), Tim Henman (ditto), Goran Ivanisevic (ranked 25th at the time but still dangerous on grass).
1999: Daniel Nestor (?), Mark Philippoussis, Tim Henman, Andre Agassi (resurgent & playing very well on surfaces at the time).
2000: Jonas Bjorkman (doubles specialist), Jan-Michael Gambill (overhyped journeyman), Vladimir Voltchkov (journeyman), Pat Rafter (nearly great).
2001 lost to 19-year-old Federer in the 4th round.
On this evidence, anybody who says Sampras played in an exceptionally strong grass-court era is deluded. On the other hand, you can't call it a weak era: Becker, Agassi, Ivanisevic, Rafter were all very good grass-court players, & guys like Henman, Philippoussis, & Pioline could all play on the stuff.
Lets do the same with Federer:
Feliciano Lopez (avarage player), Sjeng Schalken (underrated, but quite easy for the big players), Andy Roddick (good on grass), Mark Philippoussis (good, not great)
Thomas Johansson (decent player on grass), Ivo Karlovic (Obviously tricky on grass with his serve), Lleyton Hewitt (A former champ and good grasscourter), Grosjean (decent on grass), Roddick (good grasscourter, nearly great)
Nicolas Kiefer (decent player on grass), Ferrero (quite good on slow grass), Gonzalez (decent on slow grass), Hewitt (good grasscourt player), Roddick (same as above)
Richard Gasquet (can be dangerous on grass), Tim Henman (past peak good grasscourt player), Nicolas Mahut (one of the last grasscourt specialists), Tomas Berdych (decent, dangerous sometimes), Mario Ancic (good on grasscourts), Nadal (very dangerous on slow grass, but had not reached his peak yet)
Del Potro (junior Del Potro), Marat Safin (always dangerous, not very good on grass), Tommy Haas (good talented player on grass), Ferrero (decent), Gasquet (talented but a headcase), Nadal (one of the greats on slow grasscourts, here he was very confident on grass)
Söderling (dangerous on grass), Hewitt (past peak champ), Ancic (always dangerous on grass), Safin (past peak but having a great tournament here), lost to Nadal (a great Nadal at his absolute peak, playing his best tennis and with confidence skyhigh defeats Fed in a marathon 5 set match)
Kohlschreiber (decent player), Söderling (playing great tennis, good on grass), Karlovic (dangerous), Haas (good grasscourt player slightly past his peak), Roddick (at his absolute best almost beats him).
Judge for yourselves, I mean it is not that great distance between them. Becker was the name which maýbe in the end really made Sampras draw look more difficult.
Sampras 93>Federer 03
Sampras 94<Federer 04 (Hewitt>Chang, Roddick=Ivanisevic)
Sampras 95>Federer 05 (Becker>anything, Roddick=Ivanisevic)
Sampras 97<Federer 06 (Becker really past prime<young Nadal on slow grass+rest of the draw a joke for Sampras)
Sampras 98=Federer 07 (Ivanisevic on fast grass on level with Nadal on slow grass, rest of the draw quite easy for both)
Sampras 99=Federer 09 (Agassi on grass=Roddick on grass, rest of the draw quite difficult for both)
Result: 2 ties and 2-2, makes it a draw. But we still have:
Sampras 96<Federer 08 (losing in straight sets to Krajicek on gast grass<Federer losing in a heroic 5 set marathon final against an absolute peak Nadal on slow grass)
Giving Federer some advantage
Sampras 00>Federer 2010 (because winning a 7th title against Rafter in the final>a tournament that hasnt been played yet)
So Sampras still is the best grasscourt player of all time, for now....