For me, I find it difficult to choose between two events, both taking place in World War II.
First, I'd have to agree with a few other posters here and say the Doolittle raid, simply because of the sheer amount of ballsiness that was required of everyone that took part. They knew the risk they were taking, but wanted to make sure what they were doing meant something.
Second, I'd have to go with the Maikop Raid. This one's a bit less well-known, so I'll let Wikipedia do the explaining, because it can do it better than I can, lol.
In early August 1942, a Brandenburger unit of 62 Baltic and Sudeten Germans led by von Fölkersam penetrated farther into enemy territory than any other German unit. They had been ordered to seize and secure the vital Maikop oilfields. Disguised as men of the dreaded Soviet security police, the NKVD, and driving Soviet trucks, Fölkersam's unit passed through the Soviet front lines and moved deep into hostile territory. The Brandenburgers ran into a large group of Red Army deserters fleeing from the front. Fölkersam saw an opportunity to use them to the unit's advantage. By persuading them to return to the Soviet cause, he was able to join with them and move almost at will through the Russian lines.
Operating under the false identity of NKVD Major Truchin, based in Stalingrad, Fölkersam explained his role in recovering the deserters to the Soviet commander in charge of Maikop's defences. The commander not only believed Fölkersam, but the next day gave him a personal tour of the city's defenses. By August 8, the German spearheads were only 12 miles away and the Brandenburgers made their move. Using grenades to simulate an artillery attack, they knocked out the military communications centre for the city. Fölkersam then went to the Russian defenders and told them that a withdrawal was taking place. Having seen Fölkersam with their commander and lacking any communications to rebut or confirm his statement, the Soviets began to evacuate Maikop. The German spearhead entered the city without a fight on August 9, 1942.