I've realized that it doesn't matter so much what I listen to but rather how much. The more often I hear a song / album whilst working the better it helps me to focus. For a while now this has been pink floyds pulse concert from 1994.
For me, songs without lyrics work best, doesn't matter if it is classical or instrumental techno like Nine Inch Nail's "Ghosts". Songs with vocals that I've heard a lot are better than songs with vocals that I've never heard before, presumably because my brain isn't trying as hard to process the words as it would be if I hadn't heard them as much... but songs with no vocals at all are the best for focus, IME.
