If I could only have one firearm, it would be a shotgun; probably a 12 gauge pump. Small game, big game, birds in flight....as long as it's within a stones throw distance you will most likely be able to drop it with the correct load from a shotgun. Plus, shotguns are reasonably priced, shotgun ammunition is readily available in many varieties, it's plentiful, and it's comparatively easy to reload.

My second firearm, an AR15 style rifle along with a Spikes Tactical .22lr conversion kit (bolt assembly and magazines). .223 is very easy to shoot, small enough that you can carry a ton, accurate at longer distances than a shotgun, and packs enough punch to take care of just about anything in North America with well placed shots. With the conversion kit one can quickly adapt the rifle to shoot most .22lr ammo with reasonable accuracy for use against small game, or if you just don't need the full power of .223/5.56.

My choices are an attempt to cover the widest variety of situations though. If I knew where I was going and what I was doing in advance, I would probably choose something more tailored to the situation at hand.

Another tool that I think is often overlooked is the bow. Bows are quiet, have no harsh recoil to worry about, and have been used for a long time to hunt game in the Americas. As another upside, your ammunition can often be retrieved or you can make new ones with stuff you find in the wild, in some cases. Personally, for an unknown survival situation, I would probably take a good bow setup over most .22lr rifles.