Castlevania IV.
Great music that perfectly matches each level. In particular I love the playful music on the dancing ghosts stage, the ominous melody-free music on the level where you enter the castle (the level that has no boss and serves as a transition between the outside and inside), the gold level (where every item is money) and the last 10 minutes of the game. The use of sound in those last minutes is incredible, you have the fights with Slogra and guy-whose-name-I-forget, then the fight with Death, then the near silent walk out of Death's room where the lamps light up behind you as you pass them with a "woomph" sound, and the subdued confrontation with Dracula until the first stage theme reprise kicks in. An then finally the contemplative ending music.
Maybe not the best music in a CV game in terms of melodies, but IMO the best as far as matching the music to the game and using music to shape the total experience.