Win 7 on main system. Win Server 2008 R2 on my server.
A couple years ago, after some marathon benching I discovered that WinXP ran 2D benches better than Win 7, but Win 7 ran 3D benches faster than WinXP. I still have that stripped down WinXP .iso on a DVD in case I ever get back into 2D benches.
I also have Win Server 2012 beta, Ubantu, and Win Home Server on my server system.