I'm on Windows 7. It's the best OS ever. I've used every version of Windows since 3.1, OS X and several flavors of Linux. I won't be leaving Windows 7 for a long, long time.
I used XP for several years ( I skipped Vista), but no way that I'm going to go back to the XP after W7. W7 is the best.
XP still, its the most stable as far as i knew. the only prob is that its just vulnerable to viruses.