Most people would say the English Pitbull is the strongest breed of dog, but the Rottweiler has the highest recorded bite pressure of a domesticated dog.
Different dogs are stronger in different ways than other dogs. Such as, Rottweilers and Pitbulls (and similar breeds) are stronger in the jaw and neck muscles than other breeds. They are bred to protect and fight (Or originally were bred for these purposes). Alaskan Malamute, Chinook, etc; are stronger in the shoulders and hindquarters and are breed for pulling. By sheer size, Pitbulls (all types) and Saint Bernards could be considered the strongest primarily due to the fact of their weight.
However; if you were to face a dog, you would probably be less afraid of the huge breeds (such as a Mastiff), and more troubled by the smaller breeds. The larger a dog is, the harder it is for them to speed up and slow down. So although a Mastiff, once running, can inflict a lot of damage, they are by no means agile. A Pitbull, etc. or even a Rottweiler is a by far more fearsome foe.