That would depend on the person and their preferences.
For me the Windows Phone is the best choice, I prefer Microsoft products because im just so immersed with Microsoft. My phone, tablet, PC, cloud storage, server, development software, and even my school work is all somehow affiliated with Microsoft. So with me it would be logical for my next phone to be a windows phone.
Although that doesn't necessarily make it the best, For other people the iPhone is a much better option or even android.