Probably my Ruger 10/22 Takedown. It’s extremely light, extremely accurate, and so I could break it down and put it in a back-back and carry years of ammunition. If push came to shove, the last thing I’d want to do is be carrying around a big giant weapon with heavy ammunition that I’d have to leave behind. I could keep it with me for years. I could hunt with it, I could defend myself with it, I could survive with it. If I had to survive with only one weapon, it would be a good .22.
Anything else is too heavy and bulky and wouldn’t even last a year, and I couldn’t possibly carry the ammunition to last very long.
9mm or shotgun or centerfire ammunition would be way too heavy to carry in an extended survival situation.