Due to a recent PM exchange with CalBear and Adam, I decided to post this question for the rest of AH, who probably all have answered this question a thousand times:
"What if the German Panzer divisions were available for immediate action at Normandy?"
As I understand it, not only did Hitler prevent the German release of a counterattack for what had to have been days, but von Rundstedt's plan to have a more elastic Panzer reserve was shot down by Rommel, who wanted his armor close to the beaches. So basically, assume the following: Hitler's subordinates give the go-ahead to Rommel as soon as the invasion begins, instead of waiting until the evening.
Also, the reserve formations are immediately activated in order to counterattack, instead of days or even weeks after the landing. Also, consider the two versions of the Panzer reserve:
1.) A more flexible Panzer reserve further inland. The problems with this are that Allied air superiority would result in casualties in transit, and the allies might gain too much of a beachhead anyway before the reinforcements arrive.
2.) However, Rommel's version with the tanks being close to the beaches meant that IRL, two of his panzer divisions were stuck at Calais waiting for the invasion that never came. They eventually arrived, but only after Hitler finally gave them permission to be moved. Eventually reinforcements arrived and counterattacks happened, but too late.
My Assessment: If the Panzers are deployed as in IRL and counterattacked, they would have taken horrendous casualties but driven the Allies into the sea. The Soviet Union would have then won the war on the Eastern Front anyway, taking large pieces of Europe in the process.
If the Panzer reserves were activated earlier in version 1, the German formations would take heavy casualties, but would keep Allied forces bottled up in their beachhead longer (the Allies would still land of course). The storm that would happen two weeks later would damage the Mulberry ports being used by the Allies, hampering their ability to resupply their troops. The Germans at that point, would strike, defeating the landing. Soviet Union still wins the war, although it would take somewhat longer and result in the Russians controlling less of Europe than above.
#2 is trickier if only because if Rommel had gotten his way entirely, the bulk of the German forces would be located at Calais. The Allies would have quickly broken out of Normandy and defeated the Germans. If the location of the landing was predicted correctly, then the Allies would have gotten slaughtered. War continues until Germany is nuked.