Has to be the Pennsylvania rifle. Flint lock, .40 to .60 caliber, 40 to 50 inch rifled barrel, full hardwood stock, open sights. No other country found it necessary to develop and then use this weapon and it served very well indeed from about 1760 to about 1840, performing some truly amazing tasks well and reliably. Some were issued to the British ranging units in the French and Indian War and the fur trappers of the far west depended on them to keep their bellies full and their topknots secure. Sure, 3 to 5 minutes of angle is no big deal now, but back then, compared with smoothbore fowlers and military muskets, the long rifle was a wonder. Fliintlocks are much more reliable than most people realize; they can be fired upside down and even in the rain if some care is taken with the pan, and the locks are not nearly as delicate as they seem.

And then the genius Browning invented the 1911 pistol.