I've got a 90lb boxer. He's an awesome watchdog, as he barks loud and deep and looks scary through the window. Gentle as a bunny with my 10 month old son, too. Not a good attack dog, though no dog is a good attack dog unless trained as such.
His problem in a SHTF situation is that he needs a coat during our Maine winters, otherwise he'll freeze to death.
That being said, i can't see myself with any other type of dog besides a boxer.