We rescued a Carolina Dog, also known as the American Dingo.
They are a good "medium-size" dog - about 50-60 lbs - not so big that they eat you out of house and home, but big enough to be an actual deterrent.
Very pack-oriented, they bond extremely well with the family, are fiercely loyal and protective. The downside of that is they are so social that they suffer from extreme separation anxiety. When we adopted him, we were assured that he was completely crate-trained. And at the shelter, surrounded by a dozen other dogs also in crates, he had no problem. But left ALONE in the house while my wife and I were working, he literally CHEWED HIS WAY OUT OF HIS CRATE and then popped out a house window to go outside and find us. He would go back in the house when he didn't find us immediately outside. We solved the separation anxiety problem with another trip to the shelter and adopting a boxer-beagle mix - a lovable companion but no real protection.
The Carolina Dog has always been very alert (although he's 14 now and going a bit deaf). Not barking at every little noise, but you knew that there was something out there by the way he'd perk up and start listening closely. Whenever a "stranger" came onto the property, he'd adopt a very aggressive stance - head lowered, teeth bared, growling, hair on his back standing up - but he also acknowledged my position as the alpha by staying just in back of me, waiting for me to do something first.
One night when I was taking out the garbage, he sprinted ahead of me to the trash cans in the dark. I heard cans being knocked over and then my dog trotted back to me in the light - and I saw that he had a goddamned BOBCAT in his mouth, and it was pissed off! He had caught it just right between the shoulder blades where no matter how much it twisted and clawed it couldn't touch him. When he did drop it, it RAN up a tree as quick as it could to get away from him. I went over him with a fine-tooth comb to make sure he wasn't hurt. I was worried about rabies, but the bobcat hadn't managed a single scratch on my dog. Brave and loyal, he kept me from getting clawed up that night.
If you can cope with the separation anxiety, pound-for-pound you won't find a better protection dog.