tough choice, really... but I voted for the Stalingrad / Caucasus disaster. The Germans never got the oil they were looking for in the first place. Conquering Stalingrad was a stupid idea too. Conquering Moscow and eliminating the bulk of the Red Army that ready for its defence would have been the most important job for the Germans in 1942.
The easy success in South Russia was a result of the mere fact, that the Russian were not expecting it. They thought Hitler would try to take Moscow. By invading Stalingrad and Caucasus, the already thin ressources were spread on a much longer frontline, causing the siege of Stalingrad and the hastily retreat from Caucasus.
Thus my vote for this campaign as the greatest of the many blunders of Hitler...
The other options:
Eliminating British troops at Dunkirk would not have taken Britain out of the war. Sea Lion was never a real option, as Hitler aimed at territories in the East and not at conquering London.
Given the bad weather in early summer (April/May) in Russia in 1941, Barbarossa would have faced big problems, if started earlier.
The conquest of Moscow in 1941 would have been a serious blow to Russia, but remember: Napoleon reached Moscow and still didn´t win. Same is true for Germany, Stalin would have continued fighting as all the factories were at the Urals.
Conquest of Malta instead of Crete: Yep, a big mistake, but even the conquest of Cairo wouldn´t have changed that much as the Allies still could have come back to North Africa via Marocco (as they did with operation Torch). Of course, if Turkey would have turned against Russia, things could have changed seriously.
Failure of sending tanks early on against D-Day. Due to their overwhelming air power I believe D-Day would still have been a success. A bloodier one for the Allies, but once started, there was hardly a way back to the landing crafts.
Just some thoughts on that vote. Curious about other opions.
Christoph