I have chosen a Springfield Armory M1A (standard, loaded, with SS barrel) in .308 (7.62) for my bottom line SHTF rifle. The .308 has the power, range, incapacitation ability, penetration, versatility of ammunition, and availability of ammunition to make it my choice. I have mounted on it an EoTech Model 552. I also have a good inventory of spare magazines, ammunition, spare parts, armor tools, and maintenance manuals. Part of my decision on the .308 cartridge was based on my military experience where it can be just as important to shoot a thing (car, door, wall, etc.) as well as a person.
Yes, there are other calibers available, but I worry about the availability of the standard “Sporting” cartridges (.270 Win, .243 Win, etc) in a situation where everything has gone down the tubs.
My choice of the Springfield Armory M1A was based on my personal preferences. Any other .308 caliber rifle / carbine would also be a good choice, such as an Armalite AR-10, HK 91, or the DPMS FN-FAL.