Go a Rottweiler. Have had 23 all up, most were sold off as guard dogs but kept 4 in total. As said, an untrained dog is useless as a guard dog, regardless of their size. But if you know what you're doing, or are willing to pay someone to train them for you, then you'll be sweet. We used to train them to maintain a perimeter, and not cross the front fence line unless motioned to, would only allow someone into our compound with a member of the family present, and as soon as that person walks back out the gate, the dogs would attack them if they tried coming back in unattended. So all in all, well trained, pretty smart, were remarkably good with children. Etc etc.

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Air is for pussies.