In modern guns two of the most popular/ideal/efficient calibers are .22rf and .30-'06. The one only kills little things & paper, but is widely available, economical, accurate and pleasant to shoot.
For those who like to kill Big Things .30-'06 does right well. Amazing variety of bullets & developed loads, accurate.
If you are looking at muzzle loaders in similar fashion, might be good to look at what mix of paper & flesh you wish to pierce, are economics and shoulder damage a factor? Round ball or minie?
If I wanted to kill a large animal that could do me harm, if it so chose, I sure would like that .62 caliber. And Round Ball rather than minie for more effect on the beast. That last was British big game hunting experience, I believe.
For paper or steel depends on range, how serious one is, and shoulder condition. No doubt a big ball at highest tolerable velocity can make excellent groups with good retention of velocity.
Is not .50 caliber popular for chunk gun shooting these days?
In a lighter offhand rifle, some of us more fragile types prefer a rifle in the .36--.54 caliber range, for a More Pleasant Shooting Experience. My Grandfather told me he could shoot the head off a pheasant with his ~.36 halfstock percussion. He did some other things that made me inclined to believe him. Won a lot of turkeys at turn-of-the-last-century matches. Didn't use it on black bear, though, & I suspect he got pretty close to that pheasant/groundhog/whatever.
I like Jameson Irish whiskey . . .