Of course different units have different functions, but I am going to contradict my earlier statemtn a little. I think I can say some units are better than others. The easiest way to decide which units are the best is what unit you would most like to remove of your opponents.
If you are Germany it is easy to come to the conclusion that Russia’s most valuable piece is infantry and vice versa (sorry MysticalRommel).
The other countries are a little more difficult but I think it is safe to say that if you were the axis it would be best for you if the GB and the US could not make transports. You might think infantry for them as well, but they could still transport tanks. Without transports it would force them into the strategic bombing strategy.
For Japan it has to be infantry, because they start the game with a couple of well protected transports. And without transports they could still do okay with IC’s.
So in an almost complete reversal of my earlier post I am going to say these are the best units.
Russia - Infantry
Germany - Infantry
UK - Transports
Japan - Infantry
USA - Tranports