Win2000
Been running Windows 2000 on my big machine, and except for a crash or two generated by 3D Studio, I haven't had a lick of trouble with it. I can even play the Myst 3D games on it, although I don't play any of the other graphics games that sometimes demand Win98. Like others here, I'm going to give WinXP a little time.
Just loaded Win2000 onto one of my old Thinkpad 770s, and I'm going to be interested to see how it works. Had Win98 there before, with few problems.
I'm not sure that you can still buy Win2000 through normal channels. I haven't checked in a while.