I listen to way too much music so I don't have one song, but whenever I need something to focus I listen to Stella by Kashiwa Daisuke. It's long and supposedly haunting, so beware. It's probably my favorite song ever.
Otherwise, my range goes from breakcore, glitch, post-classical, hip hop, rap, post-rock, and way back to breakcore. Some Japanese music here and there too. Damn, I didn't realize it was so much.