I feel that it really depends on what each axis side is doing
For example: Germany decides to go full sealion. US should build up a “Liberation force”. Or if Germany is a lot more threating, then the US should build up a substational force and land it where ever its needed. (NA, Italy, Norway, etc.)
If Japan is more threatning, then a large fleet build up is the way to go. Droping 2 dudes on Hawaii per turn is a great idea here.
Personally, Japan is the one that I ususally go for.
Germany first buy -
1 Carrier
2 Transports
1 Destroyer
1 Sub
(Placed in SZ 101)
2 Infantry
(Western US)
2 left
Japan first buy
2 Carriers
1 Battleship
Balanced(ish) buy
1 Carrier
1 Bomber
(SZ 6)
1 Destroyer
1 Sub
1 Transport
(SZ 101)
1 Infantry
(Western US or Eastern US)