Vista Home 32bit. Despite it's bad reputation, I haven't really had any dramas with Vista - well no more problems than any previous versions.
But my OS has been installed for a couple of years now, so it's really starting to slow. Next install will be Windows 7. I'm thinking about going 64bit version too so the system can access more than 4GB of RAM. Most software now is compatible with 64bit
Desktop: Win 7 Ultimate 64Bit
Laptop 1: Vista 32bit (Only because W7 wont show DVD Drive for some reason)
Laptop 2: Win 7 Ultimate 64bit
Server: Windows Home Server
Tablet: Maemo 5