Back when I was a kid and only had 2 or 3 irons to choose from, "favorite" probably would have been easy. Since I now have a rack that is fairly well spaced out from small to large it is a case of "favorite according to task"....

Little cartridges for little jobs.
Bigger cartridges for bigger jobs.

Any one of them is the "favorite" according to the task at hand:
.20 air rifle for backyard-in-town plinking
22LR for out of town plinking
223 for prairie dogs (close)
6MM for prairie dogs (far),
270 for coyotes/antelope
308 for bench targets
338 for deer
375 for elk,
416 for something new

It all depends on what the quarry/task is when I'm heading out for fun or for a hunt, but that is just me.

Good Luck and Good Hunting,
EKM