According to Massad Ayoob, one of the most respected names in law enforcement, the two dog breeds criminals fear the most are Dobermans and Rottweilers. I've owned a German Shepherd, Doberman, English Mastiff and Great Dane, all are great for defense, although the first two were the best for that. Besides the ones mentioned there, I also like and recommend a Rhodesian Ridgeback and Anatolian Shepherd. Rottweilers and Anatolian Shepherds can be very stubborn and hard to train though. So I'd say Doberman, German Shepherd and Rhodesian Ridgeback are your best bets. There is a couple of things to keep in mind though. All of these breeds are highly active and require A LOT of both physical and mental exercise. Also, what is probably the most important, is that you need to know that the dogs you've chosen to scare other people off are not going to scare you. Every dog out there when placed in a new setting will constantly test its boundaries until it knows where it fits in in the hierarchy of the pack and what the rules of the house are. If you adopt a fully grown doberman and he gets on the couch, for example. Lets say you tell him to get off and he growls at you. Are you prepared to yank him off the couch? What if he tries to bite? Are you prepared to give commands and enforce them? If you think you are and you are prepared to spend the time and money on giving these dogs the attention they need and the exercise, then pick one of the three breeds I mentioned and you'll have both a great protector and companion. Hope this helps. E-mail me if you need more info.