Use subs/naval bombers to defeat England
I think it goes without saying that sizing up your opponents, and predicting their strategies is the key to success in MP.
As the Germans, the biggest variable is the Soviets. You will know, based on the type of player controlling the Soviets, right off the bat if you should concentrate on Navy/Air (defeat Allies first strategy) or ground (defeat Soviets/neutralize Allies strategy).
That said, any competent SU player would be what Die defines as "the backstabber," and you must be prepared for a Bigglesworth-type to take over later - even if one doesn't start in the slot. Therefore, the best German strategy should always concentrate on defeating the Soviets first, and neutralizing the Allies.
I think the definition of neutralize is a tough one to define however - in my opinion it includes driving the UK out of their homeland.
I am a big supporter of the submarine tactics - yes, all it is is an annoyance to the UK - but it helps prevent the UK from spreading her resources evenly to the Allies.
But subs alone will not sufficiently slow down the Allies to the point of neutralization.You must find a way to augment them.
With the time associated with building a strong surface fleet, obviously that option is ruled out by default unless the Soviets are vanquished.
With the cost of strategic bombers - 1 bomber equals 3 infantry divisions, not to mention research - I'm not so sure I support the bomb England down while also trying to prosecute a war against the Soviets. The primary reason I say this is they are not mutually supporting of your submarine fleet.
In my opinion the number one overlooked production strategy aspect are Naval Bombers - with no Royal Navy England is a prize waiting to be had. What you do is get the Royal Navy running roughshod trying to catch your subs. They divide their force and get into small "hunting" groups. then, when they finally find a few subs poking around off the coast of Norway the 20-stack of Naval bombers hits them.
Bam, no more hunting group. This is how you wear down the Royal Navy - and agan, no Royal Navy, no England.
Defeating England will not win the war, but it will certainly neutralize the Allies - permitting you to concentrate the rest of your resources on the SU.