If the sole question is ammo availability, the "old standbys" '06, 308, 270, 243, or 30-30 will probably be the best bet. "versatility"? More loads? Poppycock! STANDARDIZE! Pick ONE load, LEARN it in your rifle. Comes TEOTWAWKI, the time for ballistic proctology is gone!
The "exotics", 7X57 Mauser, 6.5 X 55 Swede, 7-08, .260, 25-06, 257 Roberts, 35 Whelen will be harder or impossible to find. Brass and bullets will (have) become as hen's teeth.
The answer to most of the problem is reloading: The .308 is the parent cartridge for the 7mm-08, .260, .243, and probably some I have forgotten. The 30-06 can provide .270, 25-06, and 338-06 brass.
Considering the 30-30 was THE high-powered cartridge of the 1800s, and depending upon residence, few of us will ever be called on to dispatch Grizzly bears or feral Brahma bulls, so...the .243 makes a lot of sense, as does the more-versatile .308, which can be called upon to dispatch even those two... if a tall, sturdy tree is close at hand.
But...for those who have one or more of the "exotics", "putting some supplies by" gradually, as they can be found, makes a lot of sense.