For fun at the range? 22lr
For stopping home invaders? 12 gauge
For stopping predators (2 or 4 legged) at distance? 7.62 NATO
For CCW? 9mm
For varmits? .223 or 22-250
For do-all that will see you through thick and thin without cleaning or worry? 7.62x39 in an AK.
.45 ACP, because of availability. Otherwise, 10MM. If I faced a gunfight I'd grab the 10MM and hope I don't run out of ammo, but it shouldn't take too much.
152mm... yeah, I can see that...
Lately, my favorite round has been the .44 Special... (which is, in my mind, the revolver version of the .45 ACP, my other favorite).