Assuming the dog will be OUTSIDE, I'd like to suggest a beagle. I have one. They're extremely smart, loyal, protective, and loving. Ours weighs about 60 pounds. While they're not the biggest dogs, they're all muscle, and could take out anybody. They howl, or "sing" as we call it to alert you of something. Some people are annoyed, but I think it's beautiful. Ours would make an intruder wish they'd never come within a mile of our yard.