Windows 7. My dad sent me a copy of W8 and Office a few years back while he worked at MS (no, he did work for Xbox :p), but I've never bothered to install either. I know W8 can be customized to function as a faster version of W7, but I've never seen it as worth the energy. Really should get round to install Office though...
I'm interested in W10, but i plan on waiting a few months to see how things turn out for it.