According to the book 'The Intelligence of Dogs' by Professor Stanley Coren, which breed is the least intelligent?

The Afghan Hound was placed 79 and last by Stanley Coren in his 'The Intelligence of Dogs' book on dog intelligence.

Stanley Coren, a professor of canine psychology at the University of British Columbia in Vancouver. The book was published in 1994, explaining Coren's theories about the differences in intelligence between different breeds of dogs.

When Coren's list of breed intelligence first came out there was much media attention and commentary both pro and con. However over the years the ranking of breeds and the methodology used have come to be accepted as a valid description of the differences among dog breeds in terms of the trainability aspect of dog intelligence. In addition, measurements of canine intelligence using other methods have confirmed the general pattern of these rankings including a new study using owner ratings to rank dog trainability and intelligence. In total 79 ranks are given (plus 52 ties), for a total of 131 breeds ranked.