It is hard to pick a definitive answer but according to one site, which claims to be well-regarded and credible, the Alaskan Malamute is the largest dog breed.
The Alaskan Malamute (Akmaláte) is an example of a landrace created by indigenous people in North America. The AKC recognizes it as part of its Group 3 – Northern Breeds. They are the largest purebred in that group.