LOVE me some game OSTs. I've actually wanted to bring this topic up for one of our podcasts. Honestly, there's very few that I've really listened to (this really is hard now that I think about it), but I've been doing so a little more lately.
Here's my 10, not necessarily in any particular order:
1. Persona 4: The Golden
2. Persona 3: FES
3. Frozen Synapse Prime
4. TES: Morrowind
5. Halo 2 Soundtrack (one of the best game soundtracks ever, this coming from a non-fanboy... worth picking up!)
6. Chrono Trigger (the live band arrangement here is amazing:
https://www.youtube.com/watch?v=iEGniXc7Xtc )
7. Donkey Kong games in general
8. Castlevania games in general
9. 999/Virtue's Last Reward
10. Fire Emblem: Awakening
Honorable mentions for TWEWY, Danganronpa, Disgaea, Anna, Deus Ex: HR, and Mass Effect.