I'll admit that I've never seen the entire 1999 wimbledon final but I'm not sure I'd consider if Sampras's best performance based on the youtube highlights I've seen of the match. I know the highlights don't tell the whole story, but you'd think, if anything, the highlights would make a match look more impressive than it actually was, but I just don't find sampras's level of play particularly impressive. Agassi served horribly and didn't return very well and sampras - while certainly not giving agassi anything - seems to just serve huge then volley away the returns (many being somewhat difficult volleys, to be sure). So while sampras definitely played very well, I'm more impressed by the solidness of the performance rather than anything outstandingly brilliant. I don't think it was better than Mac's 1984 wimby final performance or some of Kajiceks's grass performances, or some of Fed's 2006 wimby performances (his wins over gasquet, henman, ancic, bjorkman are all up with the best grass performances i think, and I certainly find the youtube highlights more impressive than sampras's - and as i said, if we're talking about best performances ever you're gonna want the youtube highlights to look damn impressive), also I think in the late 70's borg had a couple of dominant wins too - not too sure of the actual quality of performance on those though.
The 1999 TMC final on the other hand (which I did see) is the best I have ever seen sampras play. He was a winners machine from the baseline, and he definitely played above himself in the backhand department. Heaps of winners from that side. I do, however, think I have seem a few better individual performances than sampras in that match. As always, sampras's defence wasn't up there with the greats (even in the 1999 tmc final), and while he certainly didn't need it to be to beat agassi that day, if he had been playing someone with better movement than agassi, someone who could have returned more of his attacking shots with great depth (a nadal, hewitt, djokovic kind of player) he might not have looked so great.
I think the best individual performance ever is federer def. hewitt 2004 us open final, mostly because of the first set. best set of tennis ever played. Every forehand federer hit, it seemed, landed on hewitt's baseline and came with ridiculous pace. Federer was hitting winners from ridiculous positions - and i don't mean from way out of court, but from deep balls that gave him no angle he would just rip forehand winners. I find that kind of hitting the most impressive of all, when the opponent hits a good pace, good depth rally shot and you can only expect another rally shot in response, but instead there's a winner, made from nothing, and out of nowhere.