Well, that depends on how you define “most available.”
Here in the west this past spring, when you couldn’t find .223 or .308 ammo, you could find all the .270 Win and .280 Rem you wanted. The price was higher, sure, but you could find said ammo.
One of the tricks to buying ammo is like buying gold. Buy when the price is low and stock up. You could buy .223 and .308 reloading almost all along in surplus outlets. If your oppressor is wearing those blue helmets you would be more likely to liberate some useful .223 than 270. We all have different preferences but what will be available in time of emergency? EPA is trying to shut down lead production and nyc is trying to confiscate firearms so what you gonna do?