it all depends on the country in question. no one unit is best for all of them. but here’s my picks :) :
Russia: INF, sheer numbers can overwhelm the german war machine on a relentles march to berlin
Germany: Tanks/Armor, blitzkrieg, thats all there is to it
UK: trannies, i know it sounds stupid, but without them the UK can’t do much of anything
Japan: anything, japan needs all the help she can get on the mainland, it doesnt really matter what it is, just get units there quick
USA: Carrier w/ 2 Fighters, can we say smackdown?? they can obliterate japan’s navy while also serving to protect units in an amphib assault
now these are just my picks, they might not work for all your strats, so feel free to tweak them to work for you.