Stalingrad and Baku. Moscow was totaly empty at the time and the loses were not that big as in Stalingrad. In Stalingrad the soviets encircled the strongest german army and destroyed it. It was the only army which could do offensive operations. Plus the oil fields of Baku were essential to Germany.

The biggest factor in the Eastern Front was logistics. The infrastructure of the Soviet Union was terrific: the schorched earth tactics and the lack of normal roads plus the horrific winter wore down the German army. In winter they couldn't start their trucks because the gasoline of the trucks were frozen. They had to put a fire under the trucks. There were no concrete road, mostly dirt roads which were basically impassable in spring and autumn so the units were slowed down (and by that the supply trucks too). The supplies came from Germany and as the front moved further to the east the supply of the troops became harder and harder.

Hitler and the leadership of the army were wrong too. They (but mostly Hitler) tought they can beat the Soviets before winter so they didn't think about winter equipment. This was one of the biggest factor of Stalingrad: in winter a lot of soldier died because of the cold.