Here's my thoughts on a protection Dog / Pet.
You want a Dog that is first off a Pet, Loyal and Obedient yet fun.
Everyone here will have their opinion on what you need...
First steps should be:
http://www.akc.org/recbreeds/
http://www.dogomania.com/breed/
Look for the general temperment, adult size, grooming etc and see if it matches your needs.
You want a Dog that won't just eat someones Ass first and think about it later. You want a Highly intelligent animal with an EVEN temperment for the 99.9% of the time they won't be in protect mode. And one that will be smart enough to know the difference between a infrequent visitor to your house and an Unwanted Stranger.
REMEMBER! Dogs live up to 15+ years. Don't buy a dog on a Whim. Think about it for a Month or more.
You will be restricted by this animal 24x7x365. If you want to go away for the weekend and it can't go along you must kennel or have someone care for it.
You will need to spend extra $ for food, vet bills(shots etc) and whatever they tear up(and they will when a puppy)
My thoughts
#1 German Shepard dog
Temperament The German Shepherd Dog must be even in temperament, well balanced, self assured, totally unself-conscious and, except when provoked, totally good natured. Added to that he must be attentive and easily trained. He must possess courage, combativity and toughness, so as to be a suitable companion, watch, guard, working and herding dog.
MAKE SURE THE PARENTS ARE OFA CERTIFIED !
This relates to a few bloodlines having Hip displasia and the OFA
certification states both parents have been X-rayed and found to
be free of the signs of this fault.
If you can find a Labrador Retriever / Sheperd mix that would be the perfect Dog IMO...