Bombers. Some may argue, but I will explain:
They are useful in island game. Try to take-over a small island near enemy in the start. Bombers will be able to fly from here and bomb enemy infrastructure. Separate all cities you can from other, destroy all rails and mines you can from that place. Don't let to rebuild, bomb everything they builds and start bombing city you separated. Later transport a few troops to their continent (4 riflemens worked for me even if they had tanks and infrantry) and bomb enemy again, if you have enough bombers, you will be able to reduce everyones HP to 1. Then invade and re-base all bombers to newly conquered town. Since it's separated, enemy won't be able to quickly send armies to you. And everything they send you'll be able to bomb and kill with infrantry. Also, bombard other cities and roads around from place you have now. Later take over them and repeat everything.