That's a very subjective, infinitely discussed topic youve got there
I dare say they have strengths and weaknesses. Aesthetically iOS is better and is much better vetted, but then a little dull sometimes. Windows is lovely looking as far as I can see, but seems to have very little backing (the best apps barely seem to reach it). Android is IMO good as long as the user understands for a smart device to be excellent, 'customisability' should not outweigh design. Often simple is better.