Windows 2000 at the top

This was a pretty big leap forward and was Microsoft's last pure user OS. There are no hidden nasty features, no built in DRM, no subversive Microsoft marketing.

Windows XP second place

Microsoft committed a few sins with Windows XP that some people have forgotten. The most visual one was splitting off the deficient Home edition. Introducing DRM technologies is bad one.

Windows Vista third

Vista hasn't proved itself, so it's hard to rank. Another factor is, there are too many editions. It's DRM'd up the wazoo. A lot of features seemed to have dropped off the radar. To its credit, there have been a few welcome strides forward. Vista future is as either a Windows 98 (Watch for Vista SE), or as an XP. It's too early to tell.

Windows 95 fourth. Did the job.

Windows 98 was a cock-up. SE didn't improve things much. I always had problems with it and ended up moving to Linux Mandrake.

You've missed Windows ME off the list. I personally believe this was not only the worst OS on the planet, but perhaps, in the entire universe. If there is life out there, I very much doubt they are as stupid as us.

Windows NT and 3.1 can't really be judged side by side with the others.