The suitability of any particular survival gun is dependent upon one's individual circumstances, how one defines survival, and the circumstances which leads one to be in survival mode. In other words, is this a planned long duration wilderness adventure, or an unexpected misadventure; the result of becoming lost, or having your primary transportation sink, become disabled, or crash, or the scheduled float plane fails to appear on the day your food runs out. Mostly survival in the wilderness relates to one's ability to acquire food rather than to your ability to fend of attacks from wild animals. If a misadventure leaves you injured, this further complicates your food gathering ability. In many areas across the country, fishing gear and snare wire is the correct answer, and should always be included in one's survival kit, along with a tent, an axe and wood matches. A 12 ga shotgun is often heralded as the end all and be all of survival guns, and one must acknowledge the versatility of the shotgun and the wide array of ammo available for it, but in a survival situation the weight is finite. If the weight of 25 shotshells is compared to a brick of .22 LR, it is apparent that we can survive twenty times longer with a .22. Personally though, I believe the best choice lies somewhere in the middle.
First though, we need to dismiss the idea that hunting regulations pertain to survival, they do not, and for this reason, I believe that a small centerfire rifle trumps the .22 rimfire or the shotgun. Loaded with nonexpanding bullets, the centerfire rifle can take small game and game birds at close range. With full power soft point loads you can take migratory birds like geese or sandhill cranes at longish ranges, and big game should you encounter any, thereby increasing your food getting potential 10 fold over the traditional options. While you can't carry as much centerfire ammo per pound as you can rimfire, you can carry much more small centerfire rifle cartridges than you can shotshells, and the ability of the centerfire rifle to kill at long range trumps the other options.
Ideally the rifle is small, and the cartridge is in the .22 Hornet, .223, or the 6mm or 6.5 TCU class. A 6.5 TCU would seem to be the largest caliber one should consider given the increase in bullet weight for larger rounds, although a 6.8 SPC might be considered. The ideal platform would be something like a Contender equipped with a pistol grip folding stock thereby keeping the weight light and the bulk minimal. A compact scope and a web sling completes the package.