What is the Best Dog that will guard me yet won't bite?

i have a red nosed pitbull and shes the most sweetest thing ever. Ive had weird people come up to me in my driveway and they see her and turn right back around. their very loyal dogs if they're raised right. maybe a german shepard or rottweiler. i know a few different families with these types of dogs and their amazing to have in the family.