Short-term (a temporary crisis, like a local disaster or terrorist attack): any good, rugged rifle in .308, .30-30 or 7.62x39. I'd like an HK-91 or an FN-FAL.
Long-term (total breakdown of society): any good .22 with as much ammo as you and yours can carry. I like the Browning .22 auto and Marlin .22 lever guns, but there's much to be said for the Henry survival rifle (e. g. It floats).
An HK .308 or AR-10 without ammo is just a very expensive club, and if the Wal-Mart is never going to be open again, that .22 will still be shooting when the AK has become a doorstop.