Windows 8 has the worst interface of them all.
I have to agree with that statement a thousand times over! I don't usually hate anything, but have developed a strong hate relationship with my Win8 laptop (more like portable desktop at 17" LCD and hot i7 CPU, plus dedicated graphics card).
I think that most OS GUI's can be configured to look better or worse than their original state, so this poll is hard to answer. Of the several different OSes I have here, currently I think that MorphOS and MacOSX are my favorites for visual beauty, with AmigaOS3.9 running the AmiKit enhancements as a close 3rd place contender. Any of them can be modified to a point where they could easily switch places with the others, and that is where AmigaOS and other Amiga inspired OSes shine brighter than most other mainstream operating systems.
Some of the Linux distros I have seen over the years have been fairly good looking too, and even WindowsXP and 7 were not bad looking.
It is hard to believe something as ugly as Windows3.1 ever became popular enough to dominate, but then it was not the looks, or even the functionality that drove it's popularity to the top.
Sad that the person setting up this poll did not list all of the different Amiga inspired choices first, before running out of available choices.