Windows 7! It doesn't really matter which edition. In my opinion Ultimate is a bit over-kill though.
Windows 8 (in desktop mode) looks too much like
Windows 7 Starter. Everytime I have a driver crash and loose
Windows Aero, I'm impatiently fixing the problem. Thats how much I hate the look of Windows 7 Starter and Windows 8. Lets not even mention how
tacky looking the
Start Screen is.
I've heard there are many enhancement in Windows 8, but if they come with crippled cosmetics, I can live without them. I say crippled because that was a fitting term in Windows 7 when Windows Aero was not included.