Windows 7, because I use it for work every day, I've got all the development tools I could ever need, everything just works and is easily available.
People give Windows a lot of crap, but I haven't had a serious issue with it since about Windows 2000. It's pretty solid, if not sexy.
I've got at least one machine with pretty much any given OS, AOS3, Haiku, NetBSD, Linux, MorphOS, OSX, Amithlon (I call it an OS), you name it. Only cost is preventing an OS4 machine (hint, hint)
I like to keep my options open and love just trying new OSs.
