I concur with MacTheKnife having seen the highlights of that epic match between Borg and McEnroe.
However the best match I have seen in my life time is the Aussie Open '05 Semifinal between Federer and Safin.
This is my personal favorite. Some of the best tennis it has ever been my pleasure to watch.
Good posts, man there's been some great ones. Another of my favorites was Agassi-Samparas 2001 USO. 4 sets, all 7-6, no breaks of serve and Pete wins it in four.
Ah, this was a total heartbreaker for me. I stayed up until it was over. Great match though.
MacktheKnife, I wish I'd gotten to see that Borg/Mac match live. I've seen replays, of course, but since I already knew the outcome, it wasn't as exciting as it could have been.