What If Scenario

If the Germans put the extra effort they needed to take Leningrad, they might have won the war.
Here is why:
1) There would have been a solid front with the Finns all the way to the Arctic Ocean.
2) The Russian Navy in the Baltic would cease to exist.
3) The port of Leningrad could be use a major supply route and the network of roads and railroads out of there, especially the direct line to Moscow, were the best in the USSR at that time.
4) Not only would that take the pressure off the other more vulnerable land supply lines into Russia, Leningrad could be used to stage, or become an axis of a renewed assault on Moscow.
5) The Germans would also be able to cut off Murmansk and Archangel, blocking off those two lend lease ports.

I am no military science person, but Hitler's arrogance, brutality and foolishness doomed the Germans. if nothing else, the Germans could have played all the Stalin and Communist hating Russians off against the Soviets if not for the racist stupidity of the Nazis.