Nice idea for a thread.
Live I can think of:
Wimbledon 2007 Rd 16, Court no 18, watched Tsonga thrash Feliciano Lopez in straight sets, at the end of the match Spanish fans sitting next to us literally shrugged their shoulders and acknowledged Tsonga was too good! I was like, why haven't I heard of this guy before, he looked like he was grand slam material that day, I've been following his progress since then.
French Open quarterfinal 2009 on Court Centrale - Fernando Gonzalez thrashed Murray in 4 sets (Murray did well to win a set). I've been going to live tennis since 1995 and have all seen all the top players and legends, I still think Gonzalez has the biggest forehand I've seen live, the crowd loved it and were on his side against Murray.
Wimbledon 1995, Rd 16 Court 13 (now defunkt) - Cedric Pioline beat Kafelnikov in straight sets. Pioline was the Richard Gasquet of his day, he came up with amazing shots and was so stylish, Unlike Gasquet though he did get to two slam finals so does have something to show for his incredible talent.