Am currently listening to the Halo track since it has been quite a while since I've played the game. It's not bad, I wouldn't call it exceptional simply because it "feels" ever so slightly arrogant in the way that it's put across which detracts from it. I think it's nice music on it's own but combined with the game it didn't wow me.
My favourite soundtrack, with little doubt in my mind, is Deus Ex. Alexander Brandon clearly was not asked to "make us another atmospheric bit", he instead had a knowledge of what he was making. Every piece of music suits the situation perfectly. There are no bits where you feel bad about yourself because you aren't doing something dramatic to go along to the music your listening to.
If you're in a more subtle area such as Liberty Island or Hell's Kitchen then the music will appreciate that and give you a subtle yet still recognisable and enjoyable tune to match. Whereas if you are in an area where getting caught could mean death then the music will not embarrass the situation with some slowly building drama piece, it will instead hit you hard with something that invigorates the situation and makes you appreciate the severity of what's just happened.
The music will also change as the story unfolds, it will make it come alive for you musically. There is a fantastic moment of horror in this game marked by the piece "Enemy Within" [my favourite piece in Deus Ex]. I do not wish to ruin the exceptional story of Deus Ex but, at one point of this game, the shock of what is happening, combined with the fear of getting caught is flawlessly portrayed in this outstanding piece.
As I say, having a game with a good soundtrack is different to a game with music in it that happens to be good. If a game can combine the music into the situation and atmosphere of the game then it has achieved something great in my mind.