I think that Germany could have won the Eastern Front. First of all, if they had launched Operation Sealion in July 1940 when they had tremendous air superiority and momentum from previous victories then they could have knocked Britain out of the war, preventing them from sending supplies to the Soviet Union. This also would have made it harder for the United States to supply them.
Even if they hadn’t done that they still could have won if Hitler had not believed that he was an infallible military commander. Hitler stretched his line way too thin without many supplies, trying to conquer everything at once. The best, and probably most costly example of this is when he split Army Group South into Army Group A under List and Army Group B under Weichs. He then sent the larger forced down into the Caucuses in an attempt to reach the oil fields and the smaller force to Stalingrad. They found some of the oil fields burning and never even reached their ultimate destination, the Baku oilfields.
Also, Hitler sacked many of his good and experienced generals such as Rundstedt, for retreating at times where it made perfectly good sense. This brings up another point about Hitler’s no retreat order, which was stupid and cost many lives.