I'm still using XP. I've heard alot of good things about Windows 7 though, will probably upgrade to that within a couple months.
Windows 7 Ultimate. All the Way. It is amazing and fast. Much better than vista(disappointing) If you don't have, you should get it.
XP Pro on my desktop and laptop. Still waiting to hear from a lot more credible sources that W7U is more stable/faster/less a mem hog/ etc...
XP all the way on my new desktop as well though on my old desktop with xp and laptop with vista on it. DON.
I have been using XP from the get go, still havn't fully tapped into it so no need to explore other OS yet.
I have different variations of windows I just got 98 again to run some old programs I wrote and needed to convert. I have a lap top with 2000 pro and XP on the desk tops.