Trust me, from someone who owns a VERY protective dog, I would not recommend trying to find one....I think if you just want a dog that looks scary when you are walking down the street, you can get a german shepherd or a mastiff or a rottweiler. Those breeds can look intimidating walking down the street but are very often sweethearts once you know them.
Having a protective dog definitely limits where Duncan and I can go and what we can do...it's not easy and has been a struggle from day 1 even with all the help I have received here and professional training on my own.
I think most dogs will naturally protect their owners if someone attacks them and most protect their homes also.
However, almost all larger dogs shed and most need lots of exercise. Maybe one of the giant breeds like a great dane? I have heard they are couch potatos

Why don't you just go to the shelter and let a dog pick you?