Portal had interesting background music, but Still Alive I hate because it's always a reminder to me how the best parts of games that need to be experienced first-hand will always be spoiled the day before the game comes out by some console monkey. I'm actually annoyed that it's up there. /hate post
Halo had the opening choir music, chilling in a sci-fi way we haven't heard since 2001: A Space Oddysey, and then there was the "Halo song", you know the one. Those are the only two that stuck out in any way. The soundtrack to that game was really, completely amazing when I looked it up; too bad it's not memorable since in the game I never heard it over explosions and Elites and whatnot. The cutscenes were too boring to hear it either.
Kingdom Hearts? Are you kidding?
Metal Gear Solid had the most memorable music of all of these, par none. Of course, that's not indicative of the music quality, but that's pretty up there too. With the theme song they suckered us in, and then finished cementing it in our brains afterward with the Game Over song. Definitely one of the best musical set pieces in gaming history. Now most of the actual games... meh.
For me it was Silent Hill 2 because I'm a fanboy.