Lot of people suggesting the 30-06. That's the problem
it's too popular, everyone will be buying up the .30-06 ammo. That's why i'd suggest .270 win. It's an excellent versatile cartridge, especially when handloaded. The .270 can be loaded with bullets as light as 83 grains (there are 75 grain AP rounds but civilians cant legally buy them) and as heavy as 180 grains. The bullet selection is quite as good as the 06, but youre more likely to find .270 on the shelf if there's panic ammo buying, IMO.
.280 rem could also be a good choice as well.