How do you determine what makes a good OS? What makes iOS vs. Android or Windows Phone vs. BB10, or any other such comparison not just about the fanboyism? Is it even possible to arrive at a scientific conclusion to this question? If we look at entire ecosystems, Android and iOS are obvious choices for buyers because of the sheer amount of apps they have available. However, what's that got to do with an answer to the question, "What's the best designed OS out of the box?"
Not going to spoil it for you.