Across the street from our former home lived a Chihuahua. One day I observed him/her chasing Max, our cat. When Max got to our driveway he stopped, turned and hissed. The dog stopped, turned around and walked back across the street.
Weeks, maybe months later the Chihuahua barked at a German Shepard walking by the little dogs house. One chomp and the little dog was done, forever.
Mastiff, though I prefer my younger son's Great Danes. For myself, I prefer mutts, but at least a 50 lb mutt.