I have always felt the album Shiny Beast (Bat Chain Puller) by Captain Beefheart And The Magic Band fits Monkey Island perfectly. It has a happy sounding piratey feel and some moments of sheer genius, but general cohesion. There are also a few moments where it gets quite dark, quite emotional or quite epic. And then it even has quite a dark ending compared to the blissed out horns of the earlier songs on the album. Personally I think the songs best suited to the game are Tropical Hot Dog Night (Track 2) Ice Rose (Track 3), You Know You're A Man (Track 5) and Owed T'Alex (Track 8), but they're all good. When I used to run MI1&2 on XP before ScummVM and the sound didn't work, I always used to listen to this.
Have a listen for yourselves:
Tracks 1-2
http://www.youtube.com/watch?v=7D7KW1sWK5M
Tracks 3-5
http://www.youtube.com/watch?v=03lBIiJeEGQ
Tracks 6-7
http://www.youtube.com/watch?v=nj3yGg899h0
Tracks 8-10
http://www.youtube.com/watch?v=wdrNNhQtj0Y
Tracks 11-12
http://www.youtube.com/watch?v=iAQimJj2HYs