Numbers-wise, it'd be the 12ga. Quick and easy to go through a bunch of ammo. I really only drag them out for a few boxes of practice once/twice per year though, followed by hunting.
Time-wise, and day-in day-out, it'd be the .54 (rifle mostly, but also pistol). Those are the guns I shoot the most, and enjoy the most, but they're not rapid-fire... I shoot @ 300 rds/yr of this caliber, and shoot them every few weeks.
Next it would be the
.270Win
.22-250
.75 (sometimes shot, sometimes ball)
22LR
I don't find the modern calibers all that challenging/exciting, so I mostly just sight in with them and occasionally hunt with them
I mainly hunt with my muzzleloaders (even during regular firearms seasons) and traditional archery equipment though.
Actually, I hunt with my birds more days/year than I do with all my firearms combined per decade.