As has been mentioned training is important. But, if your seriously looking for a guardian dog the best start IS a breed that has been bred specifically as a guardian dog. Do not buy a show dog of those breeds you want to get a "working" puppy of one of those breeds. Breeds such as Great pyranise, mastiff & anatolian shepherds or mixes of breeds like this. I know wyoming, montana ranchers will have these type of dogs or mixes to guard their sheep, goats, cattle and have great success. There are many things to concider when getting one of these dogs. Is it going to be with the chickens all the time? Am I willing to put in the training and seek help to make this dog a great protecter of chickens? Can I afford the dog food and vet bills? A Great Pyranise can weigh 140 lbs. Am I ready for a dog............ Really? Or............ do I just need a good dog to let it out with the chickens the 2 or 3 hours a day they a free ranging?
If your serious about a dog for this, figure out the role you want this dog to have with your family, chickens and everyone else that comes over. There are many choices out there depending on exactly what you want. Picking a good breed for what you want done is a great start!
the lady with 4 dogs and 4 city chickens