OK, I'll bite.
If a "Walking Dead" type scenario actually were to happen, I would be happy with virtually everything I already own, although they would be loud.
I think the best, most efficient, quietest, easist to pack ammo for weapon would be a .22LR like a Ruger 10/22 or Marlin Mod 70, with a suppressor and sub sonic ammo. (As long as the ammo would still allow the semi auto rifle to function reliably.) Just my 2 cents.