Just the presence of a big dog, especially a guard breed, will make people think twice about targeting you. Some breeds are better than others, your average mongrel might bark a bit but it isn't going to die in your defence.
Personally I'm big fan of Dobermans, they are bred to protect you and they will. They also have a great nose and will find your downed deer or boar for you.
The downside to the Dobie is the short coat, a German Shepherd is much more comfortable in cold weather and arguably more effective as a visual deterrent in the UK, all that fur makes them look big and Joe Scrote might have met a police dog before.
The Dobie was the first choice dog for Marines fighting the Japanese, able to detect the enemy up to half a mile away and very good at flushing them out of caves. Not one dog patrol was ambushed by the Japanese, and one exceptional Dobie was awarded a medal for flushing 14 enemy soldiers from an underground tunnel, one at a time, going back each time for another one.
Some info here:
http://www.vet.utk.edu/wardog/background.shtml