One of my favourite games and favourite game sound tracks is Thunderforce IV
Katamari Damacy had an enjoyable soundtrack, because it was completely unlike any other sound track I had heard
Psyvariar 2 had some fantastic music, but this might be because it is my favourite games
Any of the star wars games, but only because John Wiliams had such an excellent score for the films. I was amazed back when I was a youngster by TIE Fighter - the cutscenes, the game and the music coming out of the PC. TIE Fighter did a good job of creating some nice music based on the source material.
In the second example we have a dire game, dreadful acting all made playable purely for the excellent music
Other soundtracks that are enjoyable : Ikaruga, Halo 1&2, Street Fighter IV, too many to mention.