I'd have a tough time picking a breed for solely SHTF scenarios. I've been close to every dog that has ever been my canine companion and when I think of them I don't want to think of them in the position of giving their lives up for mine. It would be similar to me thinking about sending my little niece into a fight. So, I probably wouldn't choose a fighter.
"Trouble! Run away doggie and save yourself while I take care of this!"

I probably wouldn't go for an enormous breed fearing that if times got too lean it might be tough to feed the dog.
I'd also stay away from any breed that has a list of well known medical problems or was considered a short lived breed.
Through my life I've had the pleasure of the company of a Collie, a Bloodhound, a German Shepherd, a Black Lab an Irish Terrier and now a Miniature Collie. All of them were great alert dogs, fiercely loyal, mostly obedient and a number of them good protectors. The Miniature Collie we have now is a little bit of a hardcase, but we rescued her from the SPCA and though she's learned a lot, she still has a few bad habits that surface from time to time, hence the mostly obedient notation.
So, I guess my primary concern would be an alert dog with good eyes and ears that would hopefully be more of a deterrent than a dog I would want to jump into danger, a loyal family dog that would listen and respond to commands. With this in mind, it's really going to come down to training because a lot of dog breeds fit this role.
I'd also think about having a breeding pair, because it isn't like dogs would be easy to come by in a long term SHTF scenario. The pups might even be a commodity.
"How can he think about dogs as part of the family and consider the possibility that the puppies might be a commodity?"

I know, I know...