It really is impossible to pick a single song, and for me even a list...
I can't even compile a list because I am too worried about missing out on some stuff.
However, a site I have discovered recently plays excellent music that really fits a work environment (or any other environment). The songs they choose might not be things you would play for casual listening, but they add to the work environment (e.g help you focus):
http://songza.com/ The working playlists that they offer have really kept me focused.