I would sat to get a German Shepherd, they are one of the best guard dogs. You don't really have to train them to scare off predators, but you do have to make sure they won't kill your chickens. Maybe a zapping collar?
Ask five people and you'll get six different answers. All my life I've had herding dogs, mostly English shepherds and collies and all were good with and protected farm animals and poultry. (My shelties aren't bad but they're too small to be more than an alarm clock...and they do scare off hawks.)
That said, my coop is Fort Knox so no one has to watch poultry at night, but when the chickens are out during the day, so is my English Shepherd and I can state that nothing has ever been taken on his watch.
centrarchid is right, it always depents on more than one parameter. So a GS is a great dog as every police officer will tell you, but if you not the type that loves to train and work with your dog it is more likely that you will be disapointed. They need a firm hand, early socialisation and lots of positiv training and this this applies to most shepherds and guard dog breeds. Livestock Guardian Dogs are great, too, but in the wrong environment they can be a nightmare.
So first think about how much time you will invest for training and care. ... and don't forget to look into the animal shelters in your region.
English Sheperd is an excellent choice but other similar sized dogs will work. Interaction between fencing and dog can be important. I use German Pointers but would use any similar sized breed if starting from a pup. Breeds I have also used include Black and Tan Coonhounds, Dalmations, Faust Hunting Dogs, and a Border Collie. All required training and most had use other than protecting birds. Other use often primary. Starting from pup takes 18 to 24 months to get you into good protection service but pups helps provide some training not always possible if starting with an adult which can be done with most dogs even when they have a history of killing chickens. Even mutts will do and in some situations are better suited to the rigors of outdoor living.
Second hand dogs can get you into protection very quickly but you may have a very intense period of training.