You may want to look into a German Shepherd. They are beautiful and very loyal dogs. Seeing some of these dogs in action "Protecting" certain buildings is amazing as I have seen first hand, as well they are great with kids.
The Important thing is to remember any dog that is poorly bred will go off. There is nothing you can do about that. That is why the importance of getting a breeder that is AKC certified. As well that the dog's blood line is verified by a vet. Easily done if you bring in the papers. Many breeders will say that and create some papers on the computer. A vet can check with the AKC and know forgeries on a dogs papers.
Here is a great article on guard dogs
UNPARALLELED DETECTION: Imagine an alarm system that travels with you, is not easily inactivated, and interprets what it detects with a brain programmed by nature to read body language. Remember that many violent crimes are perpetrated by people known to their victims. People who are likely to be admitted into your home past your current detection system and then suddenly reveal their violent intent. That sudden change is easily detected and challenged by a well bred and trained dog.
EFFECTIVE DETERRENCE: A well trained canine shows a willingness to defend his home and family with an authority that discourages violation of your privacy or safety. Most miscreants would rather not even test the commitment of such an animal with much easier targets available elsewhere. Let them pass you by!
OVERWHELMING DEFENSE: A well trained canine is a protective barrier difficult to breach. Here is a defensive system that comes to your aid when needed or summoned. It cannot be wrested from you to be used against you or your loved ones. Unlike a bullet or chemical spray, once launched, this weapon can be recalled if the threat no longer exists.
DEVOTED COMPANIONSHIP: Most of the time a trained dog is merely a faithful, loving, gentle and well behaved member of your household. Perhaps the most important contribution he makes is the peace of mind that comes from knowing that he is on duty while you and your family go about your daily routine.
The bold part is the most important. That the dog will recall when threat no longer exists. GS are the only dogs that will do this time and time again. A dog by nature will not let go when it is attacking unless it has the brain to know how to turn off this attack mode. GS can do this. Hence the number of attacks or mistaken attacks by other breeds, which again is something you want to avoid.
Best of Luck and be ready to spend some money not only on the dog but also on the training.
