I'm gonna have to go with the Brown Bess. We stole - sorry, "commandeered" - so many during the Revolutionary War, we never would've won without it. A close second would be the M1 Garand, simply because Patton once said it was the most important implement of warfare ever devised. Past that, I would go with the 1911 .45, because it had such longevity. Perhaps longer than any other weapon ever used in the US.