The music from Battlestar Galactica is one of the most underrated parts of the show. Often moody and mysterious, the original compositions set the tone for the show. There’s a song that isn’t an original song- Metamorphosis Five by Philip Glass. It’s first played when Starbuck returns to her apartment on Caprica and puts in a tape of her father playing the piece.
Here’s a YouTube video of a live performance of this song being played.