Just breaking it down into three major categories (rifle, pistol, shotgun), my personal choice would be a shotgun. Good for everything from collecting edible small game, taking large game with slugs, and self-defense.
Beyond that, detailed situational details are necessary.
And true, firearms may be a little over-rated in terms of "necessity" for overall survival. More important might be a good versatile blade, signaling equipment (mirror, whistle, maybe flare cartridges), and reliable, simple fire-starting materials.
Oh, and a couple of extra pairs of dry socks.