Like I said, Steve Vai did an awesome job on the intro. Also, some of the more moody sequences were very good. Actually, the moody soundtrack was a very good touch in the Halo series as a whole - a feel of the ancient merging with the future is always something that appeals to me. And Halo does this rather well, both story-wise and music-wise. It does not have, say, the solid quality of a Chrono or Final Fantasy game sountrack, but now and again it hits some very high points.
Actually, one of my favorite uses of Halo music is in the trailer for Halo 3. Anyone seen that? Bloody genius. With all this powerful and intense music they put in trailers these days - a tendancy that has become tiring for its cliche - they set up the trailer with a subtlety and power I had not expected.