sub 3 mech 2 tank if you g1 dow russia.
sub 2 bomber standard g2/g3 dow russia or invade london.
The gold standard of buys. Everything else is sub optimal. Why is that? For the G1 dow Germany is attacking 7 infantry, france (all in, no normandy or southern france, can’t risk losing mech/armor for 2 or 3 ipc better to let italy do it and use arty round 2 for normandy) attempting to strafe yugoslavia, sinking one of the two fleets (usually the bship 2 cruiser), sinking dd trans with 2 sub, and sub to sz 125 to prevent Russia NO.
For a standard game the 2 bombers have more potential. They sink naval, you could fly everything to southern italy and attack egypt, if allies park on syria with air you can attack it, bombers could bomb london and you can buy transports to take uk out of the game if UK disrespects you. The bombers are able to attack and take russia in later rounds unlike naval. Oh and bombing the Russians typically puts on a hurt G4/G5, which can occur before allies start flying fighters in. You do whatever attacks you prefer g1.
Since you use marines as an example, it seems you are playing g40 mod3, in which case sub optimal strategies become better… but g40 mod3 still heavily favors axis. Germany still brings in the pain train, except instead of a 21 ipc bid you get NO money. So initial allies attacks and defenses are diminished, this puts more pressure on the axis to play more aggressive, which they want to do anyway.