I have an iPhone 4S and I love it. Technically, it's my favorite because I own one and don't hate it. I previously owned a Blackberry Bold 9700 which was awesome for like 6 months before I realized, I couldn't really do the one thing I even wanted a smartphone for well: browse the Internet. In that way, I'm extremely easy to please. iOS, Android, and Windows Phone 7 all do that well. I liked the form factor of the Blackberry because I like physical keyboards but there was more than enough reason to ditch the platform.
I chose an iPhone because everything is made for it, from apps to accessories, and Apple releases 1 new model a year and supports it for 3. I'm serious, that's it. I don't care that I can't mod it as easily as Android because I'm not modding anything. Apple made a phone that's pretty straight forward. Some people don't like that kind of simplicity; some people do. The iPhone does what I need it to do and largely gets out of my way. Android and WP7 would function in exactly the same way for me so I don't have anything against those platforms. The only thing I don't like about Android phones is how quickly the vendors are to release new models and product lines but that's not a mark against the operating system which, from my limited experience, is pretty awesome.