.22lr (Everyone has one). The .223 is far and away the leader in sales of centrefire rifles and of ammo. It (the .223) has almost totally removed the .222 from the scene and the .222 only hangs on with older rifles still being used. .243 seems to be very popular as someone stated earlier in cropping country where legal culling of roos and extermination of feral pigs is required. From there it would have to be the .308.

Lots of other calibres exist but are in the minority.