Win 7 has been the best OS I've ever used, period. I never went to Vista. XP was good but it still needed a re-install every so often. I've been using the same Win 7 install since I first installed it for over 2 years (or however long I've had it lol!). You can't go wrong.
Win 8 is coming out soon. I plan on getting it right away but I won't install it yet. You can't beat the upgrade right away price for the pro version. I won't install it until my hardware is supported and I see what people say about it.