My favorite cartridge has a lot to do with the platform that launches it.
.45 ACP out of a full size 1911 Government.
So satisfying to shoot and it doesn't beat you up.
Next would be the ubiquitous .22 long rifle 'cause cheap plinkin' is good clean fun.
My oldest daughter used to like .44 Mag best but she has fallen in love with my Springfield Armory TRP 1911.
This summer I'll let her shoot them side by side and see what the verdict is.