There are so many that I love...but it will be over my dead body that I ever forget the themes of Final Fantasy 6. They say that if you took my brain out of my head and put it under a microscope, you'd see neurons arranged to look like the score of Final Fantasy 6.
I like Mercenaries 1 and 2 and Black by Michael Giacchino and Chris Tilton. I also have a soft spot for the music for the Silent Hill games. Oh, and there's the old SNES games I played as a kid (Chrono Trigger and Super Metroid to mention a few)
not a score, but: GTA IV. they spent a mint on licensing and it worked. there's so much music that pretty much anyone can find something they like, and in turn it can change the feel of the game. my personal favorite is the new age station...you can steal cars while listening to steve reich, philip glass and aphex twin.
One of the most innovative and revolutionary game scores ever, Star Wars : X-Wing, and the superior Star Wars : Tie Fighter, space flight sim games.
X-Wing was released in 1993, and was probably one of the first video games to use scripted music. The game was almost entirely a MIDI soundtrack, and everything blended seamlessly from one cue to the next. The score was loosely based on the John Williams themes.
The music engine in the X-Wing games mixed in realtime according to the events that happened. Enemy Star Destroyer? Imperial March? Incoming fighters, the music picks up pace.
This is stuff we see regularly in games now, but probably take for granted. These games were made in 1993 and 1995 respectively and came on 5 floppy disks!
You will have a VERY hard time finding the original versions of these games but they can be found. Star Wars : Tie Fighter, is still one of the best space sims ever made.
Incidentally, Star Wars : X-Wing, was the first video game I ever bought for myself.
However, the best score of the last 10 years, in my opinion - BY FAR - is the score of Bioshock, by Garry Schyman. It is a fantastic piece of work, and without it the game would not have half the atmosphere that it does. Sure, there's a lot of licensed music in the game, but Schyman's score is so well balanced, and so well scripted into the game, it is nothing less than a masterpiece. Not to mention the fact that it is about as non-cliche as you can get in today's game soundtrack world. Fantastic.
Other scores which definitely deserve a mention include:
Final Fantasy 7 - another MIDI score. (Nobuo Uematsu)
Command And Conquer - Franck Klepacki (and others)
Mass Effect - Jack Wall, Sam Hulick.
Silent Hill 3 - Akira Yamaoka.
Dead Space - Jason Greaves.
There are numerous others, but those are the ones that jump out.