Music in games is an important part of establishing atmosphere and feel, especially if we're talking feelings of dread and uneasiness.
I'm speaking, of course, of the scariest music tracks in games. Any games, really. This thread isn't restricted to just horror games.
Resident Evil: Mansion Basement
Metroid Fusion: SA-X
Super Paper Mario: River Twygz Bed
Pokemon Red & Blue: Lavender Town
Final Fantasy VII: Headless Shinra (Don't know if it's the exact name)