To be honest, I'm torn between Dark Souls, Nier, and Metal Gear Rising: Revengeance as the games that have my favorite video game music pieces.
Dark Souls has these really bombastic songs that fit incredibly well with the boss battles you partake in, though there's the occasional slow song that strikes hard on how melancholic the game's atmosphere is, including the theme of Firelink Shrine, the final boss battle music, and the choir you hear while journeying through Ash Lake.
Nier, on the other hand, is a game full of more atmospheric pieces of music, though there's the occasional fast-paced piece that highlights a boss battle or a tense situation. The theme of the final boss, the are that precedes the final boss; I could go on and on about how the music fits so well into each little facet.
MGR: Revengeance, however, comes in with frenetic music, with electronic wobbles and frenzied guitars wailing away as you do battle against a boss or a group of enemies, even. The songs themselves are great, and really get you energized to kick the living crap out of whatever it is you're facing.