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Windows 1.x and 2.x - Rubbish. DOS was more productive
Windows 3.x - The shape of things to come. Lots of apps and games.
Windows NT 3.x - Rubbish but stable (shape of things to come)
Windows 95 - Revolutionary. Can't say otherwise.
Windows NT 4.0 - Meh, better than NT 3.x
Windows 98 - 95 with a nicer UI
Windows ME - Tripe. Bug laden. Pretty though.
Windows 2000 - Best Microsoft OS ever.
Windows XP - 2nd best Microsoft OS ever. The fact that it's still lingering over 10 years after its release exemplifies this.
Windows Vista - Tripe. Bug laden. Pretty though.
Windows 7 - Pretty. 3rd best Microsoft OS ever.
Windows 8 - I sense a trend. From what I've used: The Start Menu cannot go. It just isn't time for it to be done with.
Extrapolating, then, we can look forward to Windows 9, the 4th best Windows OS ever?
(I won't say "best Microsoft OS" since I still carry a soft spot in my heart for MS-DOS 5) Edited 2012-05-18 07:59 UTC
Win98SE was the best 16-bit gaming OS. And when people often considered Win2K a better gaming OS the benchmarks typically still favored the 98SE. Win2K was over-rated for stability. And if you found the right mix of drivers then 98SE was good for a month at a time (before the clock bug kicked in) before a reboot was inevitable.