I think it was on the level "Gravemind", and in the game story the exact location was the mausoleum of the arbiter in High Charity (not the game level name). That song isn't just my favorite Halo song, it's my favorite short song. I can't really compare it to my other favorite songs though, which are about 10 minutes long (they are from the bands in my username). I don't enjoy the non-instrumental version that Breaking Benjamin did (not so much of a fan of the band); someone sings over the guitar. The best riff in that song blew my mind over and over again. I don't remember there being more than piece not done by the Halo music team, but to me it sounds like they chose well. The other music sounds to me such that if it were any better, it would be too good for a video game, so I guess it makes sense that they would choose such a good song when they didn't compose it themselves.