So seeing the comments and judging from my own firsthand experience:
Android: Easily customizable. So if you want your phone to specifically perform the way you want, get Android. Yet downside is could get laggy, not-that-good battery life and not as secure.
WP: Simple and easy to use. Smooth OS and long battery life while being very secure. Yet lacks customization, and the app ecosystem is not as good but still catching up (so you need patience here).
iOS: Quite a common phone (the "cool" phone that teens want), and great quality. Battery life quite good and a lot of accessories built for it. Also simple, easy to use and the best app ecosystem. Yet lacks customization and greatly overpriced (could find other phones with better firmware for cheaper). Also makes you unable to change phones that easy, as Apple makes you restricted to their services (iTunes), and lack of integration between your devices unless you own a Macbook or iPad.