IMO, the Colt M16A1....
Mine shoots .223 and .22LR, both on semi or full auto. Has 30-round magazines for both calibers. It has a collapsible stock and full sized stock, 12.5" barrel and 20" barrel and a sound suppressor for both calibers.
Has an endless number of attachments, including uppers from 9mm to .50BMG, flashlights, laser sights, scopes, grenade launcher and even a 12 gauge shotgun that attaches to the bayonet lug.
As such, it can be used for defense or offense, both at home or in the field. Fills the roll of a SHTF firearm for survivability.