I voted XP as well...
Windows 3.1, only used that briefly, and only played freecell or solitaire heh.
95 was just a bridge to be crossed on way to 98, which up until XP, I considered the best/my faveourite.
ME was just a horrible thing that should be removed from history.
Haven't really used Win2k, so I can't comment on it.
As for Vista, I had a copy of it installed to see if it was any good. From what I seen, hardly any software/drivers (at the time I used it) were compatible with an OS that was meant for things to "just work." Ate up loads of system resources while idle, so I got rid of it.
Anyway, XP is my choice... Feel free to tear my opinion up, it's only 1 persons opinon... Plus I may not have had the same experiences as others