Well the three breeds that I fear the most are the Chow, Rottweiler, and the Pit Bull and I don't know if they can ever not be to the point that they don't pose a severe threat to anyone but their owners and I have heard of several turning on the family members.
If I wanted a watch dog or one for protection I think I'd go for either a German Shepard or Doberman but even those dogs are no match for the Pit or Rottweiler due to, as said in a post above, their huge mouth that was bread for destruction of other animals. They are fighting dogs but some people have been able to train them, or so it seems.