The German Shepherd breed is known for their aggressive behaviors and tendencies. They are often touted for this behavior as they are good, strong guard dogs.
Doberman Pinscher
Doberman Pinschers were once common as guard and police dogs, and thus have a reputation of being intimidating and aggressive, especially towards strangers.
American Pit Bull Terrier
In general, pit bulls aren’t aggressive with people but are “less tolerant” of other dogs than many other breeds.
A 2008 canine aggression study found that Rottweilers are average in aggressiveness towards their owners and other dogs, but tend to be more aggressive than average towards strangers.
Rottweiler
Chihuahua
In almost every measure, out of the 35 most common breeds, Chihuahuas were reported as the most aggressive, especially toward bigger dogs they have not seen before.