Most likely the Mastiffs have the biggest dogs amongst all. If we are talking about the breeds, well, Mastiffs are probably one of the biggest ones in the world. English Mastiff, Tibetan Mastiff, and Bull Mastiff are some of the most popular dogs of this breed and work great as k9s. They are a loyal and protective breed and thus, serve as guardian dogs in many areas across the world.  You can find Rottweiler, Pit Bull, and German Shepherds, the most common K9 breeds. These dogs share the traits of being great protectors of the humans they are assigned to and make for great k9s.