Despite the fact that so many people want to rag on the iPhone, it's actually a pretty good phone and iOS 5 will make it even better. The UI is simple to learn and extremely consistent, making the iPhone great for people choosing their first smartphone. If you need apps, no other platform comes close. It all depends on what you're looking for in a phone.
Even though WP7 in its current NoDo state is lacking quite a bit of functionality (which is to be expected due to the short amount of time it's been released), I do prefer it hands down over both iOS and Android. Now that I'm running the Mango beta on my phone, I really can't see myself going back to either platform unless there are radical changes made.