I would have to go with a .357 mag. or .44 mag., 4" barrel revolver, because of the versatility. You can shoot rat shot, wadcutters, semi-wadcutters, fmj and jhp, all with complete reliability.
You can hunt with it, control vermin, carry concealed or use for home defence. If any kind of firearm ammo is available, you should be able to find something to feed it.
Maybe not ideal for anything, but good enough for everything.