I think i will go in with the below combos (may vary drastically depending on actual performance)
For Me (as Germany) -
Tanks, Mot Infantry and Mechanised get- RECON, Engineers, Maintenance, Logistics and SP-AA.(or motorised Art) (need Maintenance and Logistics more than the Field Hospitals for my crack troops).
Till Russia- you do not need the Logistics and Maintenance and thus you can have the AA & AT in your divisions (motorised versions) and then switch over to these units esp. for Heersgruppe Sud which needs to travel to the Caucasus and Heersgruppe Nord which has to traverse Forests etc.
Heersgruppe Mitte needn't change much, perhaps can keep the AT and lose AA for Maintenance.
Infantry get-
AA, AT, ART Battalions, Field Hospital and Recon. (why Recon? - i need to check before sending my troops, also Historically the Germans had the best Recon systems right until late 1944).
(also a few divisions without AT but having Engineers, to get into Caucasus and difficult terrain).
Mountain Troops/Marines and Paras-
Art, AA, Engineers, Recon and FH (no AT for these guys but Engineers to help in difficult terrain, cities etc)
For Holding Divisions (the ones you leave in Norway, France, Balkans etc)-
FH, AA, ART Battalions.
For - Whack a Mole divisions
Cav + MP + Art.
For static anti-partisan divisions
Art + MP.
If you play minors-
Infantry + Art + AA + AT is best combo, you cannot research more than that and you will be on the defensive most times.
For USA and USSR-
Field Hospital is useless and so, Signals can be added and maybe even Tank Support (if that is made available).