I vote for German Shepard. I've heard good things about the Belgian Malinois breed as well. Very alert and protective of family.
Here's some information on them.
http://www.puppydogweb.com/caninebreeds/belmalinois.htm
Watch-dog: Very High
Guard-dog: Very High, will protect their family and property, but will not attack unwarranted
German Shepard:
Watch-dog: Very High
Guard-dog: Very High
http://www.puppydogweb.com/caninebreeds/grmanshpd.htm
Both are very obedient and very loyal. I have a malamute/german shepard mix that is excellent. He has the best of both breeds.