Best SHTF rifle implies you are limited to only one rifle. Assuming legal considerations are not an issue (you can own as many firearms as you want/afford pre-SHTF) then that also implies you are not based out of a fixed location or vehicle with enough cargo capacity to allow more than one rifle. Since ammo resupply will probably be an issue, the ammo you carry with you will likely have to last you the duration of the SHTF events.
Those conditions pretty much limit you to a .22 rimfire. You can carry a thousand rounds in a backpack without adding too much weight to your load. Specialty loads such as subsonics for use with a field expedient silencer (2 liter bottle and duct tape) or hypervelocity rounds like Stingers can be added in 50 packs for very little weight and space. Produced at a rate of 2+ Billion rounds per year it is the most likely caliber to be "found" if an ammo resupply presents itself.
As for the rifle, I would go with the Marlin 39. Very accurate and reliable, takedown so it can be concealed easily, adequate ammo capacity, and being a lever action it can be fired almost as quickly as a semi auto. A levergun is the "All American" action, it has no "Assault Weapon" stigma so if you encounter the authorities they might be less inclined to confiscate a "squirrel rifle" than say an AR15.
Range is limited, but if you are part of a family/group one or more persons could have a centerfire rifle for longer range shots. As the "designated marksman", the limited ammo supply would be less of a problem.
Of course, for short term use, one can always "acquire" the rifle and ammo supply of one's adversaries. If faced with an organized force with standardized armaments it makes sense to use their equipment as resupply is at least a possibility.
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