I think some breeds may be more inclined to having the ability and understanding that we acknowledge as "smartness" over others, particularly herding breeds which are designed to work closely with humans. These breeds include Australian cattledogs, Border collies, and German shepherds, and have proven themselves to be very aware of subtle cues and possess an impressive capacity to remember.
But there are many dogs out there, purebred and mix breed, who save peoples' lives and serve our country in war, by either following strict orders or using their own intuition. What really defines smartness? I've never heard of a miniature poodle being able to track down an escaped convict, nor have I heard of a Borzoi being a therapy dog. It takes a special dog to be able to know how to follow a trail accurately, in the same way it takes a special dog to be gentle at all times. Dogs were bred for different things, and they can be "smart" in many different ways.