The best thing is rarely mentioned for some reason and no phone or mobile OS does it as well or comes close. It's the sms/text to speech over Bluetooth. It's brilliant and you don't have to touch a single button to get it to work. It is even better now that you can just "call Cortana".
Well my 1320 taught me to believe me in love at first sight & also to believe that love is blind. Because I love all WPs.
The only thing that I love about my phone is just everything
If I had to choose just one, I'd have to say the OS optimization. WP is so optimized that it runs like silk on virtually any hardware., that's awesome.