Perhaps the question should be re-phrase as to which breed has the strongest bite and is most likely to bite. I have to scientific research, but I think the Pitt Bull would win. Many Pitt Bulls are gentle, but many are not and there have been several cases in my city where young children and been mauled byPit Bulls. I owned a German Sheppard as a child and it was a gentle animal.
Any dog can be taught to be aggressive and a bite, from a poodle can hurt, but Pitt Bulls, I think, again no proof, can be encouraged to be aggressive more easily than other dogs and the bite is not the worse part--they do not let go, which may be the worse problem of all.