Unfortunately, just like in the real war, the most effective unit seems to be the Infantry. They have the IPM, they are used in territory trading (what I call attacks of attrition), they can be deployed as barriers to protect valuable units in the rear, etcetera.
Tanks, in my opinion, best compliment the IPM benefit of infantry.
Fighters come in third, as they can range out to hit, but cannot take land. Great defensive units as well.