Uh, do you mean an ATTACK dog, capable of taking down a man and chewing his face off?

A guard dog that just guards and alerts can be a little chihuahua if he bark loud enough.

My mid-sized terrier it a great alarm, and is not afraid to run down an intruder and bite the shit out of him. That's enough time for me to get in a defensive posture and take care of business, if you know what I mean.

However, the best dog is ONE YOU CAN TRAIN to do what you need him to do. A big pit bull who won't follow your commands is a lawsuit waiting to happen. That's why they are a problem breed.

A German Schutzhund (a Shepherd specialty breed) has shown up in my research time and time again as the best all-around "guard" dog, if properly raised, socialized and trained. But we're talking a big dog and a bigger commitment to training.

[link to en.wikipedia.org]