This one is actually a tough call.
I truly love the ease of use and pure awesomeness of the iOS platform but it does have its issues.
I currently own a 4th gen iPod Touch and would dearly love an iPhone (5 ;) but as a developer I feel a little left out of the club as there is a hefty tax before being able to tinker away developing my own applications. Namely, I do not have a Macbook Pro (or the like) to compliment my device.
The Android comes third because although there is little barrier to development, I am not overly enthused about working in Java again. I’d prefer to stick with the .NET Framework.
As such, the Windows Phone 7 looks very intriguing, the development environment is great, the phones look awesome but here in New Zealand where do I get one? Though to be honest I wont be getting one as I rely on my employer to supply me with a phone and a Windows Phone 7 device is not high on the list at this time.
So although I do love my iPod Touch and covet the iPhone to some degree, if the Windows Phone 7 would be my platform of choice.
The only phone I use at this point is a Nokia E71 running Symbian. Not my first device of choice but one I have to live with for the time being. I do still have an old Okta Touch a.k.a. HTC Touch, but it really only sits on a shelve gathering dust.