I'm a pretty big fan of these, especially Halo 2's OST, Halo REACH's OST and a lot of Halo 3's OST, they fit the games very well (for those of you who actually play the game, and not JUST the multiplayer). They're very emotional, as well as appropriate as to the levels and parts of the storyline that they're used in, I know a lot of people don't like the Halo franchise (non-conformists who hate just to hate it, people who think the Call of Duty games are different yet never actually play the campaign to know any better, and people who are crying sour grapes because they don't have an Xbox), but those who actually have played it and stopped to listen to the background (past the explosions, gunfire, and yelling), the music is actually quite good.
I'd like to remind anyone reading this that this post is about the music, NOT the joke I made above
Anyways my favourite Halo song is from Halo 2's OST, it called "In Amber Clad", it has a very mellow, very relaxed beat, with a mixture of happy/sad tones that blend very well