From: NE Pa, USA
Hmmmm... for me, playing one hex start against 4-5 AI, I *only* build rifle units when I have a port and need to defend it from possible naval invasion. The fact that rifle can get up to 200 entrenchment is the major factor there.
Otherwise, other than staff and trucks in HQs, I build all armored cars, and go to medium tanks as the game wears on (medium tanks are blind, you still need armored cars so you can see), along with artillery and trucks, fighters and dive bombers.
This is a major offensive force approach and the problem is, running out of oil. Happens to me all the time and when it does happen, my fighters won't intercept.
So, for this approach you need to get all your oil up to L2 just for a minor campaign, and typically get all of it up to L3 for major operations. Yeah, sure helps to capture enemy oil that is already up to L3.
So, yeah, early on I got a LOT of engineers running around and spending all my production on PP so I can get those resources up.
Initially, I need to upgrade some ore first just to get things off the ground but I almost never have to get ore up to L3.
Another small problem is enemy units on mountain tops, or deep in woods that are difficult or impossible for armor to eliminate. I typically take out the ones on mountain tops with artillery or dive bombers, and sometimes build SMG (sometimes with mortors) when it is necessary to go deep into woods.
The AI, typically builds a combination of units and, their infantry cannot stand up to my army of armored cars, and they don't have enough armor otherwise. If they do, the dive bombers are great at taking out enemy armor. Then of course, the AI will build flak but usually that does not affect things too much.
Oh, the one thing the AI will do that is dangerous to this approach... build a lot of AT guns. These are DEADLY against armored cars, and you gotta take them out with dive bombers or... knock their readiness down to almost nothing with artillery and then attack.