Windows 7. Win8 seems to be geared towards the tablets and such, windows 7 is the standard now for businesses. They won't be killing it off soon, as it's very expensive for businesses to set up their entire networks and don't like changing OS every time a new fad comes out. It's stable and functions well.
Most of my offices are on 7, I've even got some older plan viewing systems still using XP, not one win8 in the system.