Remington 870 in 12 gauge... This would be my "1" gun, so of course it gets a spot in the top 3. Birdshot, buckshot, slugs, flares... incredibly valuable tool for any survival, hunting, defensive, and even plinking situation.
AK-47... Reliable, I'm familiar with the system, simple as can be, and a good punch for medium game, as well as common ammo to stock up with, and plink with.
Glock 21 and a .22LR kit... Good capacity, size, and weight. .45 is very common, and .22 is even more common, and can get any small game, as well as help preserve the .45, and give me practice at the range if my wallet is feeling a little too light.