Although he worked for our Cold War enemy, and his classic rifle is generally used by folks we consider to be bad guys, his rifle design was well-suited for its purposes. Tens of millions of both the fully-automatic and semi-automatic versions of his famous rifle are still in use.
As far as firearms designers/inventors go, I would place John Moses Browning (a Mormon) at the top, for his classics which include, but are not limited to, the Colt 1911 and the Browning Hi-Power handguns, the M2 .50 caliber machine gun, the Browning Automatic Rifle (B.A.R.), and the Browning Auto-5 shotgun. I would rank Samuel Colt second, and Mr. Kalashnikov third.
My rankings are based on my limited knowledge of modern firearm history. Others whose knowledge extends further into the past may include other inventors in the top spots of fiream history.