I recommend a very small breed. My Shi-Tzu´s will alert me to the smallest of sounds, long before anyone has the chance to make it up the boobie-trapped stairwell. I will take care of defending the pack myself, rather than getting a big breed to do that for me and likely have him hurt or killed with no vets to be found anymore.

The main reason, tho is that you can sustain them much, much longer than a big one. On what a 40kg dog eats a month I can mine alive for half a year.

I really do think smaller dogs are way to go in a shtf situation.