The lab mix I once had (mixed, I think, with Great Dane as he looked like a well proportioned lab but weighed 120 pounds) was a lousy watch dog. He loved everyone. Of course there were some people who were afraid of him just because he was big...
Seems like a more territorial breed would make a better guard dog. Our sweet Akita that died a while back, though considerably smaller than the lab, was much better about protecting us and our property. But some breeds bring on the need to be careful about liability...