In all honesty, my favorite version of Windows was Windows 2000 (aka NT 5.0). It was simple, stable, and just worked.
Windows XP tacked on to the Windows 2000 core a bunch of crap inherited from Windows Me, and then to add insult to injury they made the default theme look like something from Playskool! If you turned most of that off, what you wound up with was essentially Windows NT 5.1. It still wasn't as stable as Windows 2000 however.
Windows Vista changed so many APIs from XP that initially it was a disaster, particularly when it came to 64-bit drivers (since nobody had those at first). It went better as time went on, but they had added so many ways for the interface to get in your way that it annoyed the crap out of me.
Windows 7 is really what Vista should have been. The interface is still cumbersome, but at least it's a little less irritating than Vista. If you want 64-bit Windows, it's really the only option. In fact, I try not to even run 32-bit Windows 7 anymore unless a program absolutely will not work on the 64-bit system (and there are a few of those).
Really, Microsoft's major problem today is design-by-committee. When faced with choices, they either include everything (see the Start Menu shutdown insanity, with 14 different ways to shut down) or choose something completely insane that a narrow test group liked (see Metro interface).