I know Sean has made a couple of good "Music for office workers" playlists...but I'm just looking for one selection here. Often I think people have different criteria for perfect working music, so a catch-all playlists isn't appropriate.
Anyway, I pick The Disintegration Loops (If choosing all of them is cheating then I'll go for DL II). Largely because I feel I need formless music to help me concentrate, ruling a lot of ambient electronica/post-rock out. I'm not well up on jazz and the SOTL drones are too gorgeous and distracting. So DL it is, the whole music fading away is kind of inspirational, too.