According to Cesar Millan, these are the best guardian dogs:
Known for its physical strength, protection instincts, courageousness, and extreme family loyalty. If an intruder crosses its path, a Bullmastiff will typically use its strength to knock them over or block their path.
2. Doberman Pinscher
If you have a large piece of property that you are trying to protect, a Doberman Pinscher is a great guard dog for you. This breed is incredibly fast and is able to reach
an intruder in a short amount of time.
Rottweilers are known to be relentless protectors of their pack. They are an intelligent breed and extremely loyal to their owners. Rottweilers are known to be aloof with strangers until properly introduced to them and are very quick learners
Traditionally used to protect herds of sheep, the Komodor possesses a natural guard dog ability. This breed is active, courageous, loyal and known for its dignity and strength. If properly socialized and trained in obedience, this breed makes a devoted family pet.
5. Giant Schnauzer
Giant Schnauzers are strong, powerful, and dominant dogs that require strict training. This breed requires a lot of mental and physical stimulation and requires constant attention. Giant Schnauzers are powerful, compact, and intimidating. Their extreme family loyalty makes them excellent guard dogs.
*I only listed 5 breeds, you can find more information by clicking here.