My favorite game music, I guess, is the soundtrack of Gothic 3 (a combination of orchestra and classic guitar) and Diablo 2 (really gloomy and always interesting, nothing repeats too much). The 70's music from GTA Vice City really gets me groovin'. And Duke Nukem's main theme really gets me going (here's a nice version of it:
http://www.youtube.com/watch?v=g-y0MdPXslo )
But the music that actually impressed me the most while playing a game is the music of Enclave (an old, seems like obscure, action RPG). I've never heard anything like it. It has calm, soothing music while exploring, but when entering a battle, it would fade perfectly into a harsher, strong music. The magic was that both tracks had the same tempo, melody, but the battle music had loud drums, horns, choir, and got you really pumped. The music shifts flawlessly. While playing, I couldn't even tell there were two tracks, though I felt the influence from the music. Example:
Explore:
http://www.youtube.com/watch?v=m3K4qI5eC3Q
Battle:
http://www.youtube.com/watch?v=jqDUZmVmgcI
Just listen alternatively at small parts of it and you'll get the point.
some of my favorite music in games was the songs in penumbra when:
1. You are in the big part of the old mine, with the creepy strings.
2. The music in the old storage. Everything about that level, including the music, was amazing, with the spider eater and the unknown monster. Quite possibly my favorite level in any video game, along with storage from amnesia and the offices in F.E.A.R. and the school from Condemned. Although I definitely think the score writers for F.E.A.R. couldve done a much better job.
Pretty much anything by Jesper Kyd (
Assassin's Creed &
Hitman soundtracks in particular). He composes a wide variety of music, invoking numerous feelings and setting a near limitless array of scenes, yet his music is very recognisable and consistent. The
Assassin's Creed II OST is probably the best example of Mr. Kyd's musical diversity.
The
Prince of Persia trilogy (the
SoT one) have some memorable tracks, although the 2nd game is more heavy metal-based.
In addition to the action genre is the
Devil May Cry soundtracks (
DMC4 in particular). Not very "dreamy" music (except maybe
Out of Darkness) but the fight tracks get the adrenaline pumping & make you feel like a badarse. And they're surprisingly catchy.
On the other end of the spectrum,
Mirror's Edge has some very nice 'astral/crystalline' atmospheric music. Those are the only words I can think of to describe it, but it's very pleasant.
Of course you can't overlook anime music.
Hellsing (TV series) has got some of the coolest, most universally-liked music I've ever heard, dipping into the "groovy" genres such as jazz (acid jazz in particular), classic rock and occasionally some nice synthesised tracks.
Hellsing: Ultimate (ongoing OVA series) has got some truly epic music as well, with bombastic pieces performed by the Warsaw symphony orchestra and some other battle-heavy 'modern' tracks.
Also, check out E.S. Posthumus. If "FUCKING EPIC!" was a music genre, they'd set the standards for sure.