Individual wickets are pretty hard to pick out for McGrath because he got so many batsmen out in the same way, but his most memorable performances for mine are Brisbane against the West Indies in 2000 where he took 10 for about 40 in the match and went for less than one an over, and Lords '05. Actually, that entire series against the West Indies was just magic from McGrath. Simply dominated the most dangerous opposition batsman of his time, and the hat-trick that included the 300 and the wicket of Lara is truly memorable.
For Warne, you can't really go past the Gatting ball. A lot of people dismiss it as overrated, but the simple fact that it was his first ever delivery in Ashes tests made it amazing enough, ignoring the fact that it was totally unplayable. The look of utter shock on Gatting's face is a moment for the ages. His matchwinning performances in the 1996 and 1999 World Cup semis are memorable as well, particularly the second one for the quality of the match, and there's simply countless times that Warne has come out in the third or fourth innings and done something amazing to turn the match. Just in the last 18 months there's been Trent Bridge last Ashes, the second test in South Africa where he got Ntini LBW in the dark to clinch the win, and the Adelaide test this month. Too many others to list. It's this ability to change the game with relentless pressure that makes him the best spinner ever, IMO.
The one Warne performance I'd love to see again is when he knocked over Pakistan at the Gabba in '94 or so with 7 for 23. I don't really remember much of anything about that spell for whatever reason, though I remember most of the series. Perhaps I missed it or something. Either way, I've never seen footage from it since, and it's meant to be one of Warne's best moments.