I really do love the BA-BA-BA-BUMMMMMM from the main Halo theme. It's a very compelling song that really suits the large-scale sci-fi battles that tend to happen whenever it plays.
Castlevania: Symphony of the Night had quite a few excellent pieces of music, and I appreciated how every section of the castle had its own unique music to go with the unique architecture. The Abandoned Mine music is, to this day, the only video game music to genuinely freak me out. Luckily, that's a very small section of the castle. Happily, most CV games (excluding the N64 ones) since SotN have continued this legacy.
Mass Effect, the Metroid Prime games, the Guitar Hero games (some songs anyway), the GTA series, and the LotR games also had excellent soundtracks.