Guard Dog is a dog with the ability to defend a territory or person.
Guard dog behaviour can be either trained or instinctive.
Guard dogs are different from Watch Dog where they will not only bark but also attack the intruder without hesitation.
Hence, guard dogs are more prone to attack postman and visitors if not restrain or train properly.
From some research, it is claimed that male dogs are considered better at guarding property (territorial instinct) whereas female dogs are better at personal guarding (maternal instinct).
Guard dogs listed are generally larger in size, more than 60lbs, because some small dog breeds will attack intruder and territorial but they are no way to deter relentless intruder.
Below are recommended top Guard dog list:
American Staffordshire Terrier
Malinois or Belgian Shepherd