The best matches I've seen were the Rafter-Agassi semis of 00 and 01. Agassi's serve is a bit problematic there, Rafter wasn't stretched at all in returning him (especially later in the matches), which I suppose takes away a bit from "high quality".
There was a 5 set Sampras-Ivanisevic semi (95, I think?), which was naturally serve heavy, but comfortably better quality than their two finals and both guys had chances on return (just 1 tie break if memory serves - amazing for those guys)
Federer-Nadal 08 of course, but that's along way removed from classic grass court tennis and I also felt Federer's inability to take on the 2nd serve was a minus point for his performance.
There was a 5 set Sampras-Ivanisevic semi (95, I think?), which was naturally serve heavy, but comfortably better quality than their two finals and both guys had chances on return (just 1 tie break if memory serves - amazing for those guys)
Federer-Nadal 08 of course, but that's along way removed from classic grass court tennis and I also felt Federer's inability to take on the 2nd serve was a minus point for his performance.