Windows 7, all the way. I have three machines currently, Windows XP, 7, and 8.
Windows XP is just lacking features that make 7 so great, and as for 8... I had to modify windows 8 to restore a proper start button, add a NON full-screen menu with the start menu search box and everything that Windows 7 had. That little touchscreen oriented start menu is garbage when you use a keyboard/mouse.