For me, Windows 2000 was the first solid desktop version. Far too many issues with multi-tasking and apparent random crashes with earlier versions. Haven't used Vista yet.
I would have to say it is windows xp. I am using vista also and it is not that good..
and windows 95/98 is just suck>.<'
blue screens almost all the time.
So far, I have tried all Windows released except for Vista.
What has been working the best for me is Windows XP. I had no problems with it, that's why I'm not so sure if I should upgrade to Windows Vista.