Unironically, the Jerry Can.

Simple, cheap, ergonomic design that allowed compact storage of fuel in a sturdy container resistant to impact, puncture, across a wide range of temperatures, along with maximizing the fuel gotten out of the container as opposed to older containers required a funnel or sort that were used.

While it’s technically a 30’s creation before the war had actually started, it’s still within the era of Nazi Germany.

Likely millions produced since then with the idea staying pretty much the same and being used in pretty much every country across the world.

While the supported operation had a lot of issues to put it mildly, the deception plan for Wacht am Rhein (the German operational title for the Ardennes winter 1944-45 counter offensive) was quite good.

This should not be confused with the various infiltration attempts using US uniforms or disguised equipment, those largely failed but still accomplished some modest friction inducing outcomes, but rather the efforts at concealing that the Germans were going to attempt a major counter offensive were very successful.

This broadly fell into two categories of "success" the first of which would be German actions, namely effective operational security (keeping the plan need to know, strict radio discipline) and some deceptive operations (false radio transmissions, emphasizing highly visible defensive measures and actions).

This was also paired with having a good understanding of what the Allies were thinking. The Allies did not anticipate a significant German counter-offensive, believing to some degree the German Army was beaten to the point it was incapable of such things (to some degree, they were right, the Winter fighting basically left the German forces all but destroyed in the west), and also that the Ardennes would remain a quiet sector due to the limited mobility through that area.

The result was that the American forces in front of the German offensive were almost totally surprised as the Germans had managed to conceal the signature of a major build up, and attacked at a time and place the Americans (and really the rest of the Allies) did not believe to be a likely event.

Of course, the Germans also greatly underestimated how strongly the American forces would resist, and how capable they were, which led into the whole offensive going off the rails very quickly. But at the least the deception plan worked!

During the North Africa campaign the Germans had broken a cipher which gave Rommel a strong advantage. The cipher was the Black Code used by the US military attache, Colonel Feller, in Cairo who gave detailed reports to Washington on what the British were doing in North Africa.

Also due to a lack of radio discipline and lack of code discipline the Germans made quite a lot of successful intercepts which gave them great insight in the strength and movements of the British armed forces. http://www.historynet.com/intercepted-communications-for-field-marshal-erwin-rommel.htm