Hmm, tough one.
I currently use Windows 2000 Pro, for lack of a better option.
There just isn't any other OS with support for the stuff I use for my music (HD-recording, midi sequencing, *good* virtual instruments), except maybe OSX, but I can't afford a proper Mac. Also, I play a few FPS games that just don't exist on other OS's.
I did run Mandrake Linux as my main OS for a few months, but I didn't make any music during that time, because of the crappy sound implementation.
Been meaning to dig into some BeOS music apps, just haven't really got around to it, because of some other apps I'm missing with that OS.