The early Stalingrad may work; Although the Caspian or Iraqi oil will not be completely cut, the logistical strain on Middle Asian railroads will increase, reducing the lend-lease weapons delivery through Iranian route. Also, taking the prime agricultural region of Volga-Akhtuba plain may result in Soviet Union starvation.
Failed invasion to Normandy is unlikely though. Main problem is what Germany did not have an air superiority in 1944, therefore moving a large mass of German troops to counter invasion will just result in many (may be majority) dying due bombing and strafing even before arriving to the landing zones.
If Stalin lost the Kaukasus, he won't have enough for all the lend-lease trucks and thousands of tanks his army needs. The exact details will vary depending on the exact circumstance, but the end is the same - USSR falls/negotiates a terrible peace/is pushed to cold war status along Volga or Urals. All of which are going to wipe out the Eastern threat to Germany.
For D-Day, Hitler could have brought close to 7:1 ratio of men against the Allies. Even if 50%! of the forces get shot up by aircraft, he still has a significant majority. On June 6-7, the Allies didn't have the capacity to fight that (Omaha beach almost got ruined by a division's worth, let alone a whole Army). Once the Wehrmacht comes into close proximity to the Allied formations, Allied bombers aren't going to be keen to strike them, at risk of hitting their own troops, or worse, supply ships. I'll admit, this isn't the better scenario, but I think it is good enough to give Hitler 8 months, the last option on the poll.
- BNC