I agree that the Great Pyrenees would be my top choice for a livestock guard; Yet, they would not be the choice if the dog doubles as a family pet. We own a rescue and several dogs. By far our best guard dog is an Australian Shepherd Mix male. He can spot a bicyclist going down the road a half mile away and run/bark. He also chases and barks at large birds flying overhead. The downside is that he can easily chase down and gather any chickens in his mouth that fly up and over the free range fenced area (he would kill them if I was not their to save the chicken). Our full size goats live within the chicken free range area and they deter hawks. Dogs can be great to watch over a property but not ideal chicken guards.
Our Australian Shepherd fishing in the pond

Goats with the Chickens.

Our Australian Shepherd fishing in the pond
Goats with the Chickens.