Re: why the nexus should be best?
Most Nexus devices, when released, are very good handsets for the price. Not top of the line, but nothing to sneer at either. The exception is probably the newest device from Moto. Very good specs, but we have yet to see how well it will perform in many users hands in every day use.
For me, the biggest things are, very quick updates, and like belodion said, the Vanilla Android experience. Meaning much less bloat than on other devices, no skinned version of Android, which, in turn, means less space is taken by the OS.