There are a lot of different breeds that are thought to be great guard dogs including the Rottweiler, Mastiff, Doberman Pinscher and the German Shepherd. German Shepherds have long been used by law enforcement and military divisions because of their intelligence and fierce loyalty and would be considered the best overall guard dog.
There isn't a consensus on the best guard dog breed. Bullmastiffs are sometimes said to be the best guard dogs. Dobermans and rottweilers are also high on the list of great guard dogs. Other great options are Giant Schnauzers, German shepherds or ridgebacks.
In my opinion it's a Doberman. We have one and he has been fantastic. He has taken to our kids, they were 3,9, and 11 when we got him. He is quite territorial. Best dog I've ever had. Easy to train, learns quick.
a pit bull or a bull mastiff because that's what we're getting next because they have an instinct born with them that if a stranger comes in they pin you down by your neck until u tell them to let the person off! its a very protective dog. there good with kids too the only problem is they drool so much.