I love my windows phone but I'll try to be subjective in my answer.
I used to use Android (and loved it) but the newer versions of it have become increasingly more complicated. I'm a techie and even trying to figure out how to put a shortcut to an email account on the home screen was painful when I tried to help my friend with her new Samsung recently. I don't know if that says more about me or the operating system, but in the earlier versions of Android it was truly a lot simpler.
I have never owned an iPhone but have used them enough to know how they work.
As of today I have to say without a doubt that Windows phone is the easiest to use. Granted, it is an immature OS and is still missing some simple features that the others already have, but the basic architecture of it is such that it is a treat to use and makes those missing features worth waiting for.
I love that the apps are listed alphabetically although I'm sure that some people may prefer to organize their own apps in groups or whatever. I love how easy it is to pin stuff (not just apps, but also contacts, email accounts and web pages) to the home page. I also love the live tiles and if the developer did a good job they can display lots of useful information. I also love that no matter what you do to it, in terms of customization, it always looks tidy and organized. I also love the horizontal swiping in apps to get to the next page and how they make it obvious what's on the next page by showing a sliver of it on the current page. It is truly a really intuitive interface.
To summarize, the OS design is far better than that of iOS and especially Android, but it is still at least one major update short of some must have features. You have to decide what's more important. A mature OS that is easy to use but limited by its basic architecture (iOS), or a mature and powerful OS that requires a lot of customization (Android) or an innovative immature OS that is ready to go out of the box and has the most potential for greatness (winPhone).