Ok. I can only talk of what I have seen that stuck in my mind as a great level of play. In the grand scheme of tennis things, what I have seen is squat, but I'll tell you anyway what impressed me the most.
In chronological order. Level refers to the winner, though in some cases the loser was not far behind. Some matches are blowouts, but the loser is not to blame.
I remember some footage of a Laver-Roche AO semifinal in 1969. Impressive play by Laver.
Borg at his first Wimbledon final against McEnroe in 1980.
Connors at Wimbledon 1982 against McEnroe.
McEnroe against Connors in 1984 at Wimbledon.
McEnroe at the USO against Connors that year.
Cash against Lendl at Wimbledon 1987.
Lendl at the Masterís final against Wilander in 1987.
Nadal against Coria in Rome 2005
Federer against Gaudio at the WTF 2005 (double bagle)
Federer against Bjorkman Wimbledon 2006
Federer against Roddick AO in 2007
Federer against Nadal Wimbledon 2007
Nadal against Djokovic at FO 2008
Nadal against Federer Wimbledon 2008
Federer against Andreev at the USO 2008
Nadal against Verdasco AO 2009
Nadal against Djokovic Madrid 2009
Nadal against Djokovic USO 2010
Djokovic against Nadal Miami 2011 (this is the best I've seen Djokovic play, and maybe Nadal as well even though he lost)