Mastiff gets my vote, but most of them that I have known that have been good guardians, have not been easily trainable. Their independence is what makes them good at their job. Then there is the drool, gas, and they can shed quite a bit.
A dog from a line with a strong guarding instinct would be best. Our first Mastiff was very protective of everyone on the farm, and of all small helpless creatures. She had extremely good judgement and a great temperament. If a large dog showed aggression towards her, she would knock it to the ground and get it to submit. If a small dog showed aggression towards her, she would play bow to it - she just couldn't take it seriously. When she played tug with an adult, she would toss them around the room, yet a child would win against her.
She did not tolerate coyotes around her farm. When they howled, she would let out the deepest most terrifying bark. The coyotes would move off in another direction.
She graduated from kindergarten, and basic obedience, and got her CGC, but competing with her would have been taxing.
The Mastiff I showed, and her daughter, from different lines than my first girl, are very trainable, but have a much stronger prey drive, and less of the guardian instinct. There was a coyote outside the chicken coop yesterday evening. The first time one has been close to our livestock in many years. Our old girl passed a few years ago, and in March, we lost our male who was second in line for defending. Now I am trying to tap into my 8 yo's guardian instincts, but it doesn't come as naturally to her.