For me, I'd have to say .357 magnum. Maximum loads are plenty effective. You can practice with 38 special wadcutter loads cheaply. Easy and inexpensive to reload with cast bullets. A multitude of factory loads available including shot cartridges. Any store that carries handgun ammunition will surely have a box of 38 special ammo of some sort in stock. Maybe not the best in every situation, but a good all around caliber.