ORIGINAL: Canoerebel
None of the above. Failure to adequately mobilize their industrial base to support a prolonged battle of attrition was by far the worst blunder. None of the Axis powers really tried to fully mobilize their economies until 1943---which is why their most productive year was 1944. There are many reasons for this, but all come back to the basic ignorance and prejudice of their military and national command authority.
Precisely ... pointless debate really as there was so much that they didn't do right. Could never win the U-Boat campaign as they never commited enough resources to it (and after '43 it was as good as over) and the war on the Eastern front was always the priority, could never win the Battle of Britain as they didn't have the range once the RAF pulled back to more northern bases (and didn't have the right bomber as already discussed), could never invade as the RN would have shredded their invasion force.
ORIGINAL: topeverest
MIke SCholls comments are pretty spot-on, but one must not overlook the findemental lack of leadership at the top in Italy, Germany, and Japan. There is where the critical errors occured. These are the 'Dunkirk' errors. Without a doubt, those issues proposed and discussed above certianly were all major tactical issues that impacted the war within the context of the larger issues.
Agreed ... one example already discussed - Kursk - was never, ever going to work once the Russians countered it with massive defensive lines and air superiority. One great point that sums up the whole thing is made by Murray and Millet in 'A War to be Won' - the essentially pointless V weapon projects cost the German war economy an estimated equivalent of 24000 fighter aircraft's worth of production ... says it all about their misguided strategy really.
As for Japan the Wargames film quote - 'A strange game. The only winning move is not to play.' - suits their situation pretty well perhaps?