A lot of my favorite game music comes from Alexander Brandon and Michiel van den Bos. By listening to the games they've produced the music for you can really see how their music has evolved with the technology. For those of you who don't know who I'm talking about, they did the music for One Must Fall, Jazz Jack Rabbit, Unreal and Deus Ex. Not sure what they're doing now though?
Best music I've heard lately though comes from breath of fire IV. Yeah it's an old game, but I only got round to playing it recently. It does a really nice thing where depending on what continent you're one the music is either the more classical operatic type of stuff we're used to in RPGs, or a sort of Drum and Bass music with indian instruments used.
Just check out this final boss music. It's pretty damn cool.
http://www.youtube.com/watch?v=noRxXKRDRsM