It always depends on the circumstances.

Here an example for India on Blood & Oil:
Troops 1.PNG
Troops 2.PNG

Those are both solid combos. I usually research according to national doctrine, geographical location, and opponent army composition. That said, my standard middle-end game army composition tends to have pretty much everything in it that is supported by doctrine. In my current successful Flashpoint as Cuba where I’m already done with all the actives and headed into Scandinavia, I have motorized, airmobile, mechanized, naval, and special forces infantry, TDs and MBTs, MRLS, Mobile Radar, Gunship and Elite Attack Helicopters, ASFs and UAVs, and corvettes.

nothing I always research. Most common units are ng and asfs.for europe I always go for strikes. I skip aa usually and go for sams (aa was helpful when the helicopter season was on ).
I don't use ballistic except If I'm a navy country then I go for ballistic subs very useful.

If I go navy I go for frigates and delay all anti air units except asfs.