I recently had a chance to play with a Dell Venue Pro. I must say - VERY VERY VERY VERY RESPONSIVE. I mean seriously - flicking stuff around never felt so instantaneous. Hard to explain.
So if you're in the market for a fluid UI and not much more (ie: 3rd party applications, though it is growing!), then WP7 is probably your best bet. In fact, I might consider a WP7 phone as my next one considering the Nokia hardware that is slated to come along with it.
But if you like tweaking your device to your hearts content, Android is probably your best bet for now. More form factors, huge hacking community in addition to the everyday 'Is it as good as an iPhone?' crowd.
As for iOS - Not that iOS doesn't have a wonderful community. I think iOS is a solid choice and has matured well beyond its first version. Buuuuut I think we all know you come to PPCGeeks for anything BUT iPhone advice :P
