Well for me it's Windows Phone 7. Why? Well I went from a HTC 7 Mozart to a Google Nexus S but I only lasted 3 days on an Android Device.
I have sold the Nexus and brought back into the windows phone 7 world with a LG opitmus 7.
2 main thing made me move back:
1. Windows phone 7 give you access to taking a picture by just holding down the camera button (even when the phones locked) which is needed when you have a 2 year old!
2. Windows phone 7 also has xbox live!!! I find that playing games without achievements just isn't the same.
I am now looking forward to Mango and all the new hardware that will come with it!
Long live windows phone 7!!!