For the lower 48 I would go with a 4" .357.
There was a suprisingly large black bear taken on Michigan's UP within the last ten years, the last really big grizzly taken out of the Rockies was a long time ago. At least that got measured for the books.
Point blank against mountain lion and gator I would be OK with .357 I think. Same for angry mama moose and two legged social work. I suspect .45Colt loads up to 20k psi could do all the same things in a "slightly" bigger package with "slightly" less recoil.