As pointed out, your individual strategy and play style will dictate the best division composition, but 3xINF+ART or 2xINF+2xART are both very workable. I tend to build one 3xINF+ENG and one 2xINF+ART+AT or 3xINF+AT per Corps along with the standard divisions, the former for river crossings, forests, and urban attacks, and the latter to deal with opposing armor. Remember that even "hard" divisions typically consist of mostly "soft" units, so any AT that you build will be a wasted division slot against the majority of the units that it will face. AT and ENG are both "situational", in that you don't want to build a lot of them, since they're usually weaker than almost anything else you can put in their place, but it really hurts not to have them when you do need their special capabilities.
INT is good in a defensive role, particularly in the West, where good airfields are plentiful and targets are fairly nearby, but I generally switch to building longer-ranged FTR at some point, for use in the East. You probably want either TAC or CAS for offensive capability (or both) to support your attacks by ground forces, because their ability to disorganize ground targets is pretty powerful over the course of a combat. Hitting a target that's not in combat isn't all that effective, because they regain the lost ORG and STR almost as fast as you deplete it, but it makes a significant difference on a unit that's locked in ground combat and can't replace losses. TAC has far better range, and is around 50% more effective versus Soft targets (most targets) versus CAS being 50% better against Hard (Armor) and ships just offshore. I often forego CAS, since it's a lot further between airbases in the East, where you need the range even more than the firepower, but others prefer to concentrate on single-engine aircraft (INT, CAS, CAG) to save Leadership. TAC will also improve more with techs as the game goes, versus CAS.
Unless you're targeting a specific field (like trying to develop nuclear weapons or massive waves of heavy bombers), the vast majority of the Theory tab entries are a waste. As pointed out, the 3 in the Logistics field are vital, especially for a German player looking to the East, but the rest are purely situational. In the latest expansion, most of the others reduce the rate at which Practicals decay, so if you intend to spam mass quantities of something at the expense of most of everything else (such as an all-motorized army), you might consider bumping the related Theory tech up a notch or two.
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