Windows 7 seems to be a winner. It's similar enough to XP that there's not a huge learning curve. In my experience, it has been very stable as well. On the other hand, I haven't heard much high praise yet for Win 8, and no compelling reason to consider it. Reviewers have been panning the changes to the Start menu.
(Disclaimer: I have no personal experience with Win 8).

I started out with nothing & I've still got most of it left
M60 VP160 QS8 EP350
M22 VP100
Algonquins