depending on what kind of shtf...i would stick with something that i knew would not be a problem...ammo/reliability/effectiveness.
for general shooting, something in .22LR, .223, .308, 7.62x39mm, and 12ga. is always a good time.
and for a shtf scenario, something in one of those or a combination of a couple of them might just cover most of your bases.
in a handgun, i would try to stick with .22LR, 9mm, or .45acp, as these would most likely be the easiest to get ammunition for if the shtf for a while.
my .02 anyway
but my favorite battle rifle would be a toss between a FAL or an AR10.