On GOG I have only one game, Total Annihilation, but I played a lot of games from GOG catalogue. So my favourite are:

1. Arcanum (Ben Houge) -OST not downloadable at GOG-
I never heard something like that in games, it's full of string quartet. Because of that a lot of people considers this as a shitty music. Bad that GOG don't deliver this but luckily it's available for free to download if you don't now.

2. Advent Rising (Tommy Tallarico, Emmanuel Fratianni, Michael Ploughman, Laurie Robinson) -OST not downloadable at GOG-
Very epic for me, Hollywood-like orchestral music. Again, bad that GOG don't deliver this. It could be listen from game files, if I remember correctly - all in OGG format.

3. Total Annihilation (Jeremy Soule)
Another grate orchestral music. I was disappointed that the two TA expansions doesn't give us new music.

4. Deus Ex (Alexander Brandon, Dan Gardopée, Michiel van den Bos, and Reeves Gabrels)
This music surprised me when playing game the first time. It created a very good atmosphere.

5. some Spellforce expansion -OST not downloadable at GOG-
I don't remember which it was, but I remember the music very good, especially this with female vocal. I don't know if it is available in any way.

When I write about Deus Ex I associated myself(did I use good words?) the OST from Jazz Jackrabbit 2 (also from Alexander Brandon). I'm curious why JJ isn't yet on GOG catalogue, it's great!