When I was a lad I had a Baikel .410 for harvesting game round the farm and loved it, with eley fourlong 6's there is no small game that you can't take cleanly, and if you add in subsonic cartridges and a silencer it is no louder than an air rifle with the ability to shoot flying/running supper.
For a single gun with versility I stand by my Beretta AL391, it will cycle slugs, heavy wildfowl loads and standard 6's allowing a full range of game to be shot cleanly.
If you had to feed yourself to survive for a prolonged period in the wild, I guess a .22 spring air rifle would keep you alive, no real maintenance and a tin of 500 pellets weighs nothing and will cleanly kill small sitting game.