Ecco the Dolphin, the CD version, had some really nice (if perhaps a bit dated) ambient new-agey sound. It stands on its own musically. And since it's redbook, it spun in my CD player for years.
Back in 1997, I took a summer vacation with some friends, went to Washington DC. We took a train to the Baltimore Aquarium, and while there, saw the dolphin show they had. I was pleasantly surprised when I realized the background music during the show was the main theme to Ecco The Dolphin. Someone working there had a clue.
My fiancee says when she was there a few years back, they still used the same music. I took the time to email the composer, Spencer Nilsen, at his website. I was curious if he knew the aquarium used the song - I wasn't trying to get them in trouble or anything, I just thought he'd think it was cool. Apparently he did, and he sent me a signed CD of the SEGA ECCO soundtrack!
Other cool soundtracks:
Streets of Rage 1&2, Yuzo Koshiro's 90s stuff had some nice dancehall techno sound.
MegaMan 2 and 3. Excellent stuff there. Primitive, sure, but great melody, really catchy.
Grim Fandango's on my Mp3 player. It's just so evocative of the influences behind the game.
The Westwood Blade Runner game had great music that really evoked the original Vangelis works. Sometimes, uh, just copying it, but still.
Outcast, while bombastic, had some really good stuff. Of course, it was the Moscow Philharmonic.
System Shock 2, again, great. A few themes repeated throughout, just good stuff.
Half-Life 1 was really good and suitably claustrophobic and alienated when it had to be.
The intros to the Fallout games, the use of the Ink Spots & Satchmo, that was PERFECT. The music in-game was really quiet and atmospheric, but it was nice.