My all-time favorite game score is for Super Mario RPG by Yoko Shimomura. It has a pleasant sound with nice melodies, and they helped me to really enjoy the experience of playing through the game. Need I mention "Beware the Forest's Mushrooms"?
I really like orchestrated game scores. Two examples that come to mind are Knights of the Old Republic II and Star Fox: Assault.
Most Nintendo series have music that I like, particularly the Mario, Zelda, and Star Fox series.
Most every game from Rare all had great music, but my favorites are from David Wise or Grant Kirkhope.
The Smash Bros. series is a great compilation of Nintendo's music, with plenty of good original themes as well. Just listen to Nobuo Uematsu's new theme for Brawl.