"Poppy Nogood and the Phantom Band" by Terry Riley
The B-side, so to speak, of Riley's masterful "A Rainbow In Curved Air", "Poppy Nogood" is one of the best and most interesting Riley compositions (that I've heard, at least). A nearly-22-minute drone for electronic keyboard and saxophone, the piece prefaces ambient music, drone, and all sorts of other minimalist forms. But unlike lots of ambient and drone, it moves plenty. It's a mindbending, beautiful piece of music.