1. Saiga .223, military AK converted with bullet guide, plus an adapter for using standard AR mags. Ammo and mags plentiful, legendary AK function and reliability. Preferably with quality folding stock.
2. Pump action 12 gauge (Mossy 500, Remy 870, etc.) with 18"cylinder bore barrel. Ammo universally available. Top folding stock for packability an option. 12 gauge flare rounds are a good choice if you anticipate running into zombies or need to "light something up".
3. Glock handgun, 9MM (my choice is Glock 19) not because it is the best cartridge (it is not), but because it is universally available and mags are as well. It is the "AR" of handguns, only super reliable.
4. Packable or compact .22 rifle. Ammo universally available. Makes little noise compared to others. Can harvest a wide variety of game, including deer with well placed shot.