I think there are lots of dogs that can do the job, but it depends on their personality, not necessarily their breed. If you want to up the odds considerably, go with a LGD, that is what they are bred to do. Just do your research so you get one suited for your needs. I did not want a dog that barked all night long, which some GP's are prone to do, that is how they guard, by warning off predators. I chose a Maremma for that reason. He does occasionally bark at night, if he hears or see's something, but usually just a short warning and then stops unless the threat continues or comes closer. Then I know there is a problem and get up and go see.