Crossbreed.

genetically stronger, less health problems and IMO smarter than purebreds.

for shtf you dont just want protection, youll need something with a good nose and something that can possibly hunt.

look for a mix of more than two larger breeds,try factor in what breeds have what traits(some dogs have better noses for tracking/hunting whereas say any herding dog, besides some larger shepards will have stronger visual abilities than scenting abilities.

some dogs are couchpotatoes that couldnt defend shit regardless of the breed...i had a mastiff cross that was.