Different dog breeds have different bite strengths.
Bite strength depends on the training of the jaws, the size of the dog and the level
of its aggression - which makes the question of which dog breed has the strongest
bite a fairly difficult question to answer, since each of these factors depend upon
variables specific to each dog.
Angry guard dogs, those which are forced to participate in dog fights as well as those serving in a police or military unit, naturally have stronger jaws because they are more highly trained. Quite the opposite, dogs which are big by size, but are not aggressive and have never bitten anyone (including other pets), haven't trained their jaws much and therefore have weaker bites.
Despite such variables, dog bite strength can be measured (usually expressed in terms of force per unit area - with units depending upon what part of the world such measurements are being reported in). Experiments led by National Geographic where able to determine the three breeds with the strongest bites. The experiments included letting an aggressive dog attack a human equipped with special protective sleeves which contained an integrated system to measure the strength of the bite.
Here are the results. The breed with the strongest bite is the Rottweiler, which can generate a bite force of around 150 kilos per centimeter.
Second place goes to the German shepherd, which generates around 110 kilos per centimeter. (By the way, this bite force, combined with the love and devotion of the breed, make it ideal as a guard or security dog).
Last but not least is the Pitbull. One of the most aggressive dogs, Pitbulls have a bite force of approximately 105 kilos per centimeter.
If we were to compare the strength of dog's bite with that of some other species, the result would look like this: Nile crocodile (1,100 kilos per cm), water turtle (460 kilos per cm), hyena (450 kilos per cm), lion (315 kilos per cm). It is believed that the T-Rex was able to generate a bite force of around 1,500 kilos per cm!
While those are some scary bit force numbers, it is highly unlikely that you will ever meet a T-Rex, hyena, water turtle, lion or crocodile on the street...but be wary of angry dogs, especially of they are one of the top three breeds listed above. You will probably want to steer clear of them until, at least, they've calmed down.