My vote goes to the Sten. It changed the way armies looked at firearms - they were no longer like watches, to be looked after and repaired. If a Sten stops working, you throw it away, and indent for a new one. As disposable as a plastic razor. With the massive losses of equipment at Dunkirk, the UK needed replacements FAST, and the Sten came out of the trap like a champion greyhound - from idea to working prototype in just weeks. It was heavily influenced by the iconic Mp40 (Misnamed by many after Hugo Schmiesser) but WITHOUT the MP40's "unique selling point" - the mainspring of an MP40 is packaged up as a telescopic tube; when you disassemble the gun under field conditions, what falls out looks not like a spring, but more like a bicycle pump. Helps keep out the crud. Advanced Primer Ignition was not a new idea with the Sten, but is a clever idea to have incorporated. When you pull the trigger, and the sten's bolt moves forward, picking up a round along the way, it fires that round BEFORE the bolt has fully closed. The "bang" comes when the bolt is still moving forward - and that "bang" includes recoil, starting to push the bolt BACK before it fully closes. This impacts on the rate of fire: forces it down to a controllable level. It was retained in the Sterling (sucessor to the Sten, and the hardware I was encouraged to lug around with me rather a few years later.) In 1940, the Sten filled a dangerous gap in the country's armament, but didn't just "do the job"; it did it well enough to remain in production (as the Sterling) and be exported worldwide for several decades. It was produced with a built-in Maxim silencer - which worked very well - it was air dropped in huge numbers to resistance fighters all over the world. Many of the Stens dropped on Warsaw fell into German hands, and at the end of the war were pressed into German service (as were straight copies, made in Germany called the "Potsdam aparatus".) Stens were produced in sheds and garages all over occupied Europe by the resistance - it's just THAT simple a design. When the ENEMY is copying your gear... you know it's good.