There's no such thing as a dumb dog. A breed that is not as up-to-par with the other dogs can be seen in any breed, it just depends if they have a genetic mutation. Most genetic mutation causes dogs to act differently. Sometims dogs can suffer the same genetic mutation as other humans like OCD, cancer and influenza. 

This is a hard question to answer because everyone's definition of a "dumb dog" is going to be different. Some consider train-ability a sign of intelligence while others look simply at how a dog acts around them. The Afghan Hound seems to be a dog that many consider fairly dumb. They are hard to train and have no history of productive characteristics. Bulldogs are also often considered dumb due to their laziness and stumpy demeanor. When it comes to actual mental development of a dog (not just behavior), a lot of the toy breeds like chiuauas or poodles tend to be quite stinted due to a lot of inbreeding in the "fancy" dog community.

Given the caveats from bmalc899, a trainer's definition of least intelligent are

106. Borzoi

107. Chow Chow

108. Bull dog

109. Basenji

110. Afghan Hound