For australia, i can only think of 4 that are certain.

.22 LR (just the cheapness and availability as mentioned. Is sort of the standard)

.223 Remington (Heaps of ex-military ammo, will never die out)

.308 Winchester (reason as .223 Remington, just that its bigger and more suitable for pigs and allowed to be used on deer, for people in the states i guess .30-06 is a good option)

12 gauge shotgun (when one projectile isn't enough)

The last gun choice would be purely on where you do most of your shooting. For northern Territory I would pick a big bore either .375 H&H or 9.3x74R in a double rifle although the latter is not that popular over here its very popular in Europe so is 8x68S which is also another good large cartridge.

The rest of australia, probably a cartridge in between .22 LR and the .223 Remington. Something like a .22 Hornet, .218 Bee, .17 Remington, maybe even .17 HMR.