Well, as a Fila mastiff owner, and a German Rottie owner, I can say...my Rottie is straight forward and to the point. Barks, and growls warnings...first. Brutus, my Fila will bark, at a distance...but, someone up close, no barking or growling...He is very sneaky and very quiet about it, much like a cat. I have to watch him closely. Both are velcro dogs, but my Fila has a much shorter velcro strip than my Rottie. Both are gentle loving dogs, they love attention and their family. Each would protect me.
I would suggest one of the Mastiffs(there are several) or a Great Dane. All are gentle giants in their own rite. All will protect you, all need to be socialized and obedience trained, and all want to be next to you. But, research to get the one that fits your lifestyle best. Each one has different traits that may, or may not appeal to you.
None of the above dogs would be good outside only dogs. They must be near their owner, even my Rottie requires that love and affection.