Mass Effect: Reminds me of all the 80's Sci-Fi movies I love: Blade Runner, The Terminator, Dune, Aliens. All in the best way possible. Hell, I could write a book about how awesome this soundtrack is.
The Original Diablo: The Tristram theme was just fantastic. When you got down the the laybrinth, and you heard the drum beats and the eerie violin screeching, you knew you were a step closer to hell. My favorite has to be the Catacombs music, because it just sounds so gothic and badass. The caves sound like a demon is going to pop out and anal rape you, but when you got to hell, it just got underwhelming.
System Shock 2: The Trip Hop soundtrack was spot on for the cyberpunk horror atmosphere of this game. I still love the music in the first part of the Rec Deck.
Mafia: I have never played the game, actually, but I have heard the opening titles music. It's pretty damn awesome.
The Elder Scrolls III Morrowind: This has got to be my favorite classical style soundtrack. The brilliant use of string instruments are what really got me. For the record, I thought the Oblivion musical score was kinda lacking.
The Original Metroid: They actually made an 8-bit game where the music was great and NOT annoying. Once you got down to Brinstar... ohhh... man... gives me a boner just thinking about it.
Super Metroid: 16 bit sound. At it's absolute best. I loved every bit of this soundtrack. There's nothing bad I can say about it. Absolutely nothing.
StarCraft/Brood War: Every race in the game had a great unique musical score for them. The Terrans had a rock track that might as well have been composed by the band Rush. The zerg had a shock rock, scary ambient style to their sound. The protoss had a new age sound to them, I have to say.
Fallout: The mellow, ambient soundtrack was well fitting to the darkness and eeriness of post apocolyptic life. The music that plays when you get to the Followers of the Apocolypse monestary gives me chills.
Dune 2000: The Rise of the Harkonnen is my favorite track on there. I loved the entire soundtrack actually. Good times, good times.
C&C Red Alert: Hell March and Rollout are priceless tunes.
C&C Tiberian Sun: Good soundtrack as well. Still think Timebomb is one of the best video game tracks ever composed.