It really depends on which method you prefer.
A G1 attack can catch the Russians a little off guard and take advantage of their defenses still being relatively thin. The downfall is that Germany’s offense will also be a little thin and be more prone to Russian counter-attacks.
A G3 or G4 attack allows Germany the maximum build up time before Russia can declare war and can make for a much stronger punch into Russia’s borders all along the line. I often wait until G4 because you can hit Russia on 4 fronts (Romania > Bessarabia, Hungary > E Poland, Poland > Baltic States, Finland > Karelia) and have a very strong front line after the battles - harder for Russia to counter attack. The downfall of this is that Russia also gets 3 turns of build-up so they will have more stuff for you to fight. Generally, Germany will still have more than Russia, but if the dice don’t favor you, your front lines could get really ate up by the time you reach Moscow.
A lot depends on what USA does when they get into the war. If they go 80%-100% Europe, your Russian offensive could peter out fast because you have to fend off an American invasion or try to bail out Italy. On the other hand, Japan should be doing very well in the Pacific and you may get an overall Axis win that way.
If USA goes mostly Pacific, then you should be very successful in Russia. The UK will be busy trying to fight Italy and your U-Boats while you pour tanks into Russia and steadily grind them back. However, If Japan plays poorly, it’s possible the USA could wrap things up in the Pacific early enough to still come over and cause trouble for you in Europe before you can take Moscow. Then it’s the dreaded TWO FRONT WAR for Germany.
