Here's another +1 for Mark Seibert. His music stands (IMO) as the best in the Sierra line: KQ5, as previously mentioned, as well as Codename: Iceman, SQ4, QFG1, PQ2, and the outstanding Conquests games. I also loved Chris Brayman's score of KQ6... in fact, it may be even better than Seibert's score of the fifth game.
If you get a chance, head over to Mark Seibert's last.fm page and vote DOWN the slick-haired German guy and vote for the nerdy guy in the litte picture on the left. Click on the pictures to cast your votes!