Other contributors have said "What's the least smartest dog?" is the same question as "What is the most dumbest dog?" If you believe that these are not asking the same thing and should be answered differently, click here
if you mean the type of breed: Men. They are called humans. They are rarely smart unless they were breed from albert inestine in DMR and there early stages of polio
There is no really dumb [ unintelligent ] breed. Sometimes people think Dalmations aren't too smart, but the fact is, the are very prone to hearing problems, and their people don't notice.