I have a new all-time favorite song:
0. Electronical Parade - Killer 7
This is the song that plays while you explore a children's theme park in Killer 7. For such a serious and dark game, this song is a ray of light. It nicely accompanies the fireworks and bright lights going off in the background, and conveys a really happy atmosphere. The song has such a magical melody to it (like the golden saucer from FFVII), and you really feel like dancing to it while you explore the amusement park.
That is until you find out that an assassin who owns the park is kidnapping children, harvesting their organs, and then selling them on the black market. Oh, and he rapes the girls he kidnaps.
After discovering that, listening to this song is akin to seeing Satan get a job as a birthday clown. It's really creepy, and manages to make that "M" rating stick out a whole lot more.