Special Units
Special units cannot be purchased with PUs. Each Advanced Production Center (APC) you own produces one special unit a turn. You must research tech to be able to build APCs in the first place. Also, you must research the tech for the units you would like to build.
In the above lists, heavy tanks, Panthers, King Tigers, early jets, jets, Superfortresses, and flying wings are all special units. Superfortresses and flying wings take 2 turns to build.
Whose tank tech tree is the best? That all depends on what you want. If your focus is on building tanks with PUs, Britain's tank tech is probably the best. Its Comet tank tech adds +1 to the attack and defense of British tanks. (A Comet was rather similar to a Firefly.) If your focus is on special unit tanks, Germany's King Tiger tech allows you to build the best special unit tank in the game. With the right tech, each German APC can produce a free Panther each turn in addition to its normal production. Expect late game German armies to have their share of Panthers, and the occasional King Tiger. The Soviet player has access to good tank tech. He or she can research the ability to buy tanks at reduced cost. The IS-2 tech gives an upgrade to Soviet heavy tanks. Japan is the odd man out. At the start of the game it can build light tanks only (no medium tanks). It can research tank tech to let it buy normal tanks. But no heavy tanks for Japan.
As if to make up for this, Japan's Yamato battleships are the most expensive and the most powerful in the game. A Yamato battleship gives you more bang for the buck than a normal battleship would.
