Personally, I'm currently more a fan of Android than any other OS.
I like the iPhone, but at the end of the day I just feel that Android is the better OS for getting stuff done.
Of course, as with any phone, it all comes down to the hardware AND the OS.
I've got an HTC Touch pro, but no amount of nifty hardware can help hide the shipwreck that is Windows Mobile.
Of course, it may all be useless because AFAIK Oz still does not have a carrier offering Android
The applications on the iPhone are better, but Android is catching up fairly fast, though I have heard that developing for Android is not as easy, mainly because the SDK is still in its infancy.
I am disappointed in Google though, becuase they seem to have forgotten about Android, and have yet to deliver Google earth, voice command search, Reader or a host of other Google enabled apps to the Android platform.
Even though I still love my Touch Pro, I'd suggest staying away from Windows Mobile for the time being. It is dated, and just really needs a total rewrite.
For ease of use, nothing beats the iPhone.
The browser on WM is a total turd, even when you install Opera. It just doesn't "feel" right. The browser on Android is fantastic, and alsmost as good as on the iPhone (both are built around the Webkit browser).
As a music player, the Android phones are still lacking; the current music player is very basic, and there are only 2 or 3 basic video players, nothing as nice as on the iPhone, and of course you'll also lack the integration into iTunes.