In no particular order: Portal 2, Legend of Zelda: Twilight Princess, and Diablo 2.
(I'm considering getting a minor in Video Game Scoring/Sound Design, as this is something I pay a lot of attention to.)
I think the music in Zelda is beautiful, simple as that; great orchestrations, even if they are sampled (It's a video game, that's really the best way).
Diablo 2 is excellent because of its dissonance and ambience. It doesn't have a clear beginning/end, so it works very well for video games. And it's so creepy sounding; it really creates the feel. Along with the sound of carvers in the distance. =P
Portal 2 does a really good job of incorporating the music into the game. For example, in the beginning scene, listen to the alarm; it starts out as an alarm, but it ends up being the melody of the song, seamlessly transitioning. There are lots of brilliant sound design/synthesis techniques used to get this effect throughout the game. I also geek out at all the awesome FM synthesis used for the percussion sounds.