Leningrad is somewhere in the middle since it's a bit random, unless you can hold two cities and not even give Finland a chance to join war. To do that you need some help with reinforcements, fortifications, garrisons and so. Terrain is good though with natural chokepoints and narrow corridors.
Easiest is Moscow with abundance of reinforcements and favourable terrain close to the capital. Just spam armies dig in and let them come.
What is your ranking hardest-easiest ?
I also would like to hear from German commanders about their experiences
