I woudn't call him the best simply by virtue of the fact that he doesn't play both ends of the floor, to me that is why he didn't deserve his second MVP.
In the offense I would say he is top 2-3 behind maybe Chris Paul or Deron Williams. His experience and floor vision, his scoring and passing keep him up there, but both of the aforementioned players are waaaaay more athletic than Nash, and at this level and at their age, they are better overall, and either one will school him on the offensive side AND do a decent job defending him on the defensive side. Nash is a veteran and you will only slow down his scoring for a while, so he is still deadly there, but Williams, Paul, and Baron Davis can defend him, he can't or won't defend them.