What was Hitler's biggest wartime victory?

Fort Eben Emael, May 10, 1940

Explanation:

It is one of the world's largest and most powerful fortifications which Allied war planners counted on to halt the German attack on Western Europe. Its fall was the key to the Nazi victory in the West. German units advanced to the Atlantic, trapping more than 500,000 British and French soldiers, triggering the panicky evacuation from Dunkirk and making Hitler master of Europe for the next four year.