I've worked european and aussie doberman, the europeans as simoman said are very sharp and edgy, not what i look for in a protection dog. One of the best I've ever seen was a dingo/GSD we had in at work for a while, very switched on, smart, tough, aggresive, but at the same time very biddable and loyal.
The safest bet would be a well bred german shep -simoman I've pm'd you.
rotties i find need more persuasion to be loyal and protective.
american bulldogs, or the ones i've worked with at least amaze me every time. love em.
pit bulls can be trained to a reasonable degree as can staffs and a