When my 110lb pit bull barked, you might not have known what he was, but you knew he was BIG. OTOH if you still had cojones and got in the house, he would have shown you where all the valuables were and helped you carry them out.
The point of that story is; if you want a watch dog get a pit bull. If you want a guard dog do NOT get a pit bull. My top five guard dogs;
1) Turkish Kangal
2) Caucasian Ovtcharka
3) Rottweiler
4) Cane Corso
5) Dogo Argentino
You can't go wrong with any of those guys. The Kangal and Ovtcharka are thick-furred dogs, perfect for winter and are primarily outside dogs, although they can come inside and be a wonderful family dog as well.
To show why the Kangal is at the top of my list, watch this;
http://www.youtube.com/watch?v=gTYWWqY7Lv8
I guarantee no one will break into your home when they hear one of these dogs in it.