These are my rankings based on having used all of the devices:
User Interface Look:
1. WP7 (best default interface for sure)
2. Android (Can be made to look AND function better than the others. Though most phones tend to be a vomit mess of random widgets by default).
3. iPhone (probably the worst of all and in dire need of updating. It's almost like ONLY having the app drawer as my interface on android - blegh).
Social:
1. Android/WP7 (incredible facebook integration including contact FB updates and "widgets")
3. iPhone
Gaming:
1. iPhone
2. Android/WP7 (Android may not even hold here after this year)
Picture Viewer (stock):
1. WP7
2. iPhone/Android (you can even argue that Android is number 1 what with complete picasa and dropbox integration; assuming we're using the stock nexus gallery)
Major services associated:
WP7 Zune, Xbox Live, Windows Live, Office
iPhone-iTunes, MobileMe
Android-Best version of Google Maps and Gmail
Blackberry: LOL