Out of the box,
WP8.1 is the best looking.
iOS8 comes in second.
Blackberry 10.3 in third.
Now the reason I stated "Out of the box" is because you can easily customize Android Lollipop/KitKat/Jellybean to look better than all three.
I'd argue that because of a lack of continuity across some skins and with some launchers that Android is less beautiful.
Too subjective, heck I could argue for any of them dependant on my mood even..
But, of course, if it were objective there would be no point in asking opinions.
webOS