When I considered this problem, I started by looking at inf-to-art ratio
when testing on combat-sim against pure infantry defense, (ex 12 inf + 8 art v 18 inf) I found that somewhere between 35% and 43% art (in number of units).
for the tank/mech mix it is closer to 45/55 which gives the highest attack strength/ unit.
so I try to build about 2/3 inf, 1/3 art as russia (after I have built art the first few turns), I need those extra inf for schirmishing.
for germany (in barbarossa) I find that mechs are probably the best unit, seeing as my goal is to be able to withstand a russian counterattack in rostov as soon as possible.
on the sea, i try to build as many subs as possible, once I have enough air defence to defend against an attack without destroyers, subs is usually the best unit to build, at least until 40% of your fleet is subs (when defending) and as much as possible when attacking. 3 ipc/combatdie is just insanely good.