Nokia Lumia 920
First of all, Microsoft Windows Phone OS is fun to play with. It has a great intergration with its Live Services plus others to begin with. However, the thing that really impresses me is Nokia's contribution to the platform, and Nokia is known to have great support for its devices for years (Although they've discountinued theri Symbian phones, but they are still updating their apps on it).
But keep in mind that there are some small things missing here and there (such as no orientation lock, which is killing me), but Microsoft/Nokia has been building things at a very fast pace.
Currently there isn't a lot of apps available at the Market Place, but if there are enough people using it, program developers will take notice and will start develop apps for it. And Microsoft has always been great with program developers (they gave me a phone for free, and they invite me to join their party/events from time to time), and their eco-systems are getting stronger and stronger every day (XBox, Windows 8, Windows Phone), and it is easy to port from one platform to another platform if you follow their Metro guideline, so a lot of us are expecting the apps catalog to catch on eventually.