Just putting this out there - Windows 98 ME (x64) Will attach a poll as soon as i can rmember how to do it ... (Remembered)
You don't have the most awesome interface ever: Windows 2000 Professional. I mean really, come on, if you're gonna make these polls, provide the choices that matter.
The poll is not about interfaces, it is about VISUAL! experiences 2000 was a JOY to work with, on an esthetic level but............ no bells and whistles, as with most server variants (especially this part @boyonthebus)
I found Windows 2000 Professional to be the best visual experience of all the versions of Windows ever made. I mean, the first time I enabled the shadow under the mouse pointer, holy crap, seriously, it was a big moment and it was gorgeous on a nice display. See, the problem with polls is you're just gonna get opinions since there's no way to ever say what the "best" of most anything actually happens to be - personal opinions are not facts and obviously they change constantly. But my vote still stands with Win2K Pro, just for the record, I'm simply unable to add that to the poll.
Visually Windows 10 is fine for me, what needs to be totally revamped is how search and indexing works.
In my opinion Windows 7 was the most beautiful with it's flagship Aero theme, it has looked very futuristic at the time and it still looks futuristic and beautiful even today! Windows 10 on the other hand looks minimalistic and "flat" with all those tiles, like something designed for smartphones and tablets.