I don't like the Smith & Wesson 15-22, cause the action isn't a true straight blowback AR, but it's own custom patented gun. You can't use all regular mil spec AR parts on a S&W.
But it's still a decent gun for the money.
Well the 12 gauge may be the biggest round, but it’s range isn’t very short so I would go with the 5.56 nato since it has mediocre range and the ability to do a bit more damage then a 9mm or .22
The 556 would probably win since it's the only cartridge with use beyond 100 yards. But each one is the most popular cartridge in its respective category.