Bullmastiff

• The bullmastiff received the rating of "best guard dog" from Pet Meds Online. Loyal, courageous and strongly protective, bullmastiffs are 40 percent bulldog and 60 percent mastiff with an average lifespan of 12 years. They provide protection by biting, standing in front of an intruder to block his way, or knocking an intruder down and pinning him to the ground. The bullmastiff makes an excellent companion. He prefers being with his family and does not like being left alone.
  
  

Doberman Pinscher

• Considered the second-best guard dog and the fifth smartest dog in the world, the Doberman pinscher is quick to attack when he senses danger for his family. Dobermans are fearless, extremely loyal, strong and fast and are often used by the police and military. Dobermans originated as a mixture of Rottweiler, geyhound and terrier, according to Pet Meds Online. They have a lifespan of 9 to 15 years. Some Dobermans are known to be temperamental, but a loving environment produces a loving and stable dog. Although Doberman pinschers are highly trainable, JustDogBreeds.com recommends obedience training for them.
  
  

Rottweiler

• The Rottweiler is the ninth smartest dog in the world and some say it is the best guard dog. Rottweilers who receive training at an early age make great family pets. A Rottweiler likes children, is very protective of his family and does not take to strangers. This breed has a reputation for its superior protective capabilities, bravery, strength, stamina and resistance to pain. Their average lifespan is 10 to 13 years. Your Rottweiler may require professional training for socialization to temper its natural aggressiveness.
  
  

German Shepherd

• The German shepherd is used as a police dog partly because of its high level of intelligence. They are considered the third smartest dog in the world and fast learners. Their average lifespan is 12 to 14 years old. While obedient and loyal, shepherds are often over protective. They make excellent pets and are good with children, but they have been known to bite. Avoid problems by getting a properly bred German shepherd from a reputable breeder, and begin your dog's training and socialization at a very young age.