Whats the best protection dog for outside?

The crackhead neighbors next door have been screwing with my livestock. I guess they find it amusing to open my gate when I'm not home so they'll get out. I want a dog that can endure cold temps and scary enough to deter a stranger and guard. The dog will have to be outside when I'm not home but of course inside when I am. Don't tell me to put a lock on my gate either for safety reasons. Don't tell me either its mean to keep a dog outside when its a dogs job to guard.