I still think that Windows NT 4 was the best Windows OS ever. And may be the best GUI ever. I always switch to classic theme, if I have to use Windows. In last versions it's not very classic, though.
I also loved the workstation-y, crisp UI that would let me drag a window with its contents around the screen. My computer wasn't even all that powerful by the standards of the time.
I honestly had a server running Windows 2000 untill about 4 years ago. In that time, it was only really rebooted for system updates. It's sitting in my spare room.
During my training, I worked - for a couple of months - on a web application where the test/development environment ran on a Sun machine running Solaris 8, and eventually I install Cygwin/X on my desktop (which ran, can you dig it, Windows 2000!) to start a CDE session and work from there. That grey/pink look somehow looked both butt-ugly and incredibly stylish. ;-)