I work in the veterinary field and this is my expirience with the following breeds:
German Shepherds: HIGH ENERGY. Most have a high prey drive as well. They tend to be very neurotic and out of control if you don't give them a job. Unlike a lab without a job, these dogs can become aggressive without a job. If you make protecting the home their job they will accept the task to the fullest but will probably be aggressive towards friends and family if not socialized properly. These dogs are prone to bad hips and elbows as well as cancer.
St. Bernards: These dogs are rarely the lumbering nice dogs you have seen on tv. They have a few health issues, bad hips being number one. They need a firm owner who is a good leader. [Firm but fair]. One of our good clients had two of them, one of them was a great dog, goofy and mellow, the other one started out that way but then decided she liked eating small dogs and small children. I took her on a walk one time because she had eating something she shouldn't have and were inducing vomiting. Even while feeling ill she was stalking kids on the sidewalk. One of my coworkers fostered a young st bernard and he tried to bite like 4 different people, randomly. People he knew that had handled him before. I think you need to be sure you can handle a 100lb dog that will potentially try to eat someone.
Pit Bulls: Scary looking but not very good in the guarding department as they love people. They are terriers however and can have a high prey drive. If improperly socialized they can be dog aggressive, but sometimes even with socialization they can become dog aggressive.
Rottweilers: Again, a breed that needs a firm but fair hand. I would not recommend this breed for novice dog owners. They are also prone to bad hips and cancer.
Labrador Retriever/Golden Retriever: Pretty typical dogs, if you get a dog from hunting lines they tend to be hyper and more on the smaller side. "Field Type" is something you want to stay away from. The show lines tend to be bulkier more mellow dogs.
I would really think about why you're getting this dog. You might find a nice large mutt at a shelter who would look scary enough without having some of the strong behavior treats a lot of breeds have. Also you have to train a dog how to protect you, not let them be protective because it can get out of hand really easily. I really don't have a breed recommendation for you, but I think if I was getting a dog based on size I would get a Mastiff, a Leonberger, or maybe even a Great Dane [although Great Danes can be big pansies!]. Irish Wolfhounds are also really cool dogs, but can have a high prey drive.