That is indeed a good question.  There are just way too many variables to give a clear answer unless we just say the strongest period regardless of size.  If that is the case then I would go with the English Mastiff for sheer strength.  Not considering stamina, agility, or athleticism.   Enter the CO, Kangal, CAO, Bull Mastiff - and the field gets pretty muddy. other question is how do we measure strength?