Each of the major combatants had some weapon or weapon system that was considered superior to all the competition. The Soviets had the T-34 tank and the Shturmovic ground attack aircraft. The British had the Spitfire fighter and the Mosquito fighter/bomber. The United States had the B-17 bomber, the P-51 fighter, the aircraft carrier battle group and a few other items. The Germans had the ME-109, FW-190 and ME-262 fighters, the 'Schmeisser' submachine gun and some others. The Japanese had the Zero fighter and a decent Naval Air Force. The editor gets to make the definitions and choose the things that tickle his fancy; so I'll limit the discussion here to rifles and machine guns.
I confess to some personal prejudice. I carried an M-1 during boot camp in the Coast Guard, and I own one now. I think it's a fine rifle, but we can't even begin to count it among the greatest until a persistent myth is quashed. The M-1 is a .30 caliber, gas operated, semi automatic rifle. Ammunition is supplied in eight round 'en bloc' clips that are completely inserted into the rifle, not like the external magazines used in modern rifles. When the eighth round is fired, a spring in the bottom of the action flips the clip out of the weapon with a metallic 'twang'.
The myth is that, in the middle of combat, this 'twang' would allow the enemy to see when a rifleman was out of ammo and having to reload. This allowed them to assault his position and kill him when he was defenseless.
Poppycock. If anyone can provide a documented instance of this happening, that takes into account the possibility of coincidence, I'd be VERY interested. If you've ever been on a firing line, the noise is incredible. I don't believe that any enemy further away than five feet could even hear it flipping out, especially since it and the final shot are virtually simultaneous. And see it as it jumps? My head wouldn't be up that high in a firefight. Even if he did hear it, "Where was that noise? Over there? Get him!" Oops, he reloaded already. Reloading a fresh clip takes less than 5 seconds.
I will now consider the myth quashed. You may berate me at will.
The greatest battle implemented ever devised would have to be one that had a significant impact on the final outcome, either in achieving victory or delaying defeat. And the candidates are:
