Depends what kind of SHTF scenario we're talking about but honestly while it might seem ridiculous and is so bad at so many things…
To me the obvious answer is actually gonna be a ruger 10/22 takedown, preferably stainless synthetic. Can carry thousands of rounds on you without issue, can be used effectively to procure food for a looooong time, you can certainly defend yourself with one. is not a battle rifle, a long range big game/sniper/antimaterial rifle, or a defensive shotgun but is a more versatile all purpose survival tool than any of those on their own.
Of course having to be with but one gun sounds awful, but if it came down to that, having to bug out in a hurry with one gun and ammo and go disappear in the woods for a while…the ruger 10/22 is my easy and obvious choice.