I believe your best protection is yourself.... As for dogs, I have trained all types of breeds and mutts to be protective. No, I won't take all the credit, because if you bond with your dog and provide for him/her, then most likely than not, that dog is going to protect his/her territory and your family. Stats. show that if an intuder even thinks there is ANY type of dog in the house, the intruder will seek other places.
I for one, trained my boxer to be protective and guard me, most which came natural, but I had to travel a lot and I trained him to protect my car, since I had to sleep sometimes in hotel parking lots. NO ONE.... even my family, could not approach the car while he and I was in it with out him "going off." I also trained him in physical attack....
Bottom line, you just need a good dog that you love and loves you even more. I also have a gun that has been transported over state lines... and I have it in my closet now.
So, if you really want a feeling of protection, take self defence, lock up your house, have a good dog inside, maybe a weapon, an alarm system and common sense.
I can't specify a breed, because I know of several pit bulls, rotts, german shep, that will allow anyone in the house, that's not always the answer. So don't think in terms of "scary breeds" esp. if you don't know their behavior. Good luck. Also, why are you so interesed in a guard dog? My blue and gold macaw parrot is also very mean and protects me like I am her "bit*h." lol No, she really does.