A lot of it depends on where you are.

In the united states/north america, the AR-15 is hands down the platform you want when SHTF. The way the DI system works on the AR allows it to blow must gross stuff inside the system out of it (gas tube fills hollow in bolt carrier to unlock bolt) assuming it even gets in the very highly sealed off system to begin with. More importantly, I'd argue, is the ability to find parts/ammo basically EVERYWHERE, since it's the most popular rifle in the united states.

For eastern europe/the rest of the world, you're gonna want to find an AK, but be careful since parts arent exactly compatible between platforms. Specifically you want an AK-74, because most warsaw pact countries have switched over to the 5.45 as opposed to the old 7.62.