Here’s a few of my thoughts. If playing the US I like to expand Industry in 5 pt incremants (if you increase IC in territories with 100% Infrastructure it will take less time (about 8-9 months) for the additional industry to be built. Building up your Industry not only gives you more production points to spend it also increases your Transport Capacity. Research Carrier tech like crazy & when you get to Improved Carrier (Type V i think) stop and build 15 (you can have them ready before the war starts). Create 3 carrier groups (5 CV 1 HC 6 LC 6 DD) & go kick a**. I like 15 carriers becasue you can create 3 (5 carrier) groups which are just about invincible & later you can reoganize into 5 (3 carrier) groups. Even a 3 carrier group is pretty formidable. As for all those BB & CA the US starts with, I form them into surface groups to tag along with my carrier groups. They’re useful for providing shore bombardment but are useless in a naval battle by themselves against carriers.
If you’re playing the UK, for god’s sakes take control of your allies’ troops the last thing you want is Canadian/Aussie troops mounting unauthorized offensives. If you take control of France you can prevent the fall of France (you will need to get as many canadians to europe as soon as you can).
If you playing Germany, get lot’s of fighters good ones that have a Tac bmb capability, don’t bother with dive bombers their range is too short get Tactical bombers instead. You will need the Fighters on the Western front to intercept those pesky strategic bombers the UK/US send. When you invade Russia create Tactical Bomber wings of 1 fighter & 3 Tactical bombers. Once the invasion of Russia starts watch your Transport Capacity & make sure you have plenty of “Gestapo” (garrison units with the MP attachement). Every territory that has even 1% partisan activity will reduce your Transport Capacity, an overloaded TC will reduce your resupply/reorganization of troops & planes eventually bringing your offensives to a screeching halt.
In general I go for mobility lots of tanks & mechanized infantry. Airborne units are great I like to Build 3 Corps (3 units each). Without them taking africa will take forever. They’re great for dropping behind enemy lines during an offensive, invading overseas territorys where your transports would be destroyed & providing a ready reserve that can respond instantly to a break in your lines (especially on the Russian Front where the frontline is so huge).