Win 7 Ult 64 with a tri-boot of XP Pro, and Linux. Thats my laptop.

I also have a XP Home AIO, and three macbooks. Two Pros and one Air. All running Snow Leopard.

I am still partial to Windows 7, been using it since beta testing began.

I only have the macbooks cause our company uses them and we get new ones every year. Dumb cause I only run AutoCAD and Tekla on them through Boot Camp.