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Panzer
Replaces tank, gains a +50% bonus versus armored units.
Historical rant (delete this part):
Panzer is the German term for "Armor" or "Armor plate". Proper use of Panzers was instrumental in Germany's initial success in the early WWII. The use of massed armor supported by concentrated fighter/bomber support was first demonstrated on the western front in 1940. Dubbed Blitzkrieg or "lightning war", it was repeated with huge initial success against the numerically superior Soviet Union in 1941. After initial successes, the Germans decided to commit Panzer troops in street-fighting in Stalingrad, which enabled the Soviets to envelope and destroy the largest and best-equipped German army at the time, ensuring allied victory in WWII.
While the best German Panzers were far superior to the Allies' (the German tank ace Michael Wittmann destroyed 138 enemy tanks, which would have been impossible for any allied tank commander), the Soviet tanks were about equal in performance. The mass-produced T-34 was the best tank in the world when it was launched, which led to the Germans designing the Tiger and Panther tanks. The German success were due to superior doctrine of mobile warfare (remember civ II?), operational skill, and superior gunnery at long range. The Soviet army did not have much success until it started using Breakthrough tactics. Poetically enough, the encirclement of the German sixth army Stalingrad was the first such big-scale attack
Civ IV rant starts here:
The Panzer is unique unit which takes the longest to be able to produce. You will need the techs of industrialism and rifling as well as access to oil (combustion). However this also means that in the late-game only Germany will have a qualitative advantage over other civs, as all their unique units will be outdated.
While the +50% bonus versus armored units is a sizable bonus, it will only work against tanks and modern armor. Panzers will always receive favourable odds against enemy tanks regardless of where combat is joined, due to lack of defensive bonuses. Against scattered small stacks of enemy tanks, this should lead to the German player being able to kill enemy units while only sustaining unit damage. This will lead to a great win/loss ratio, and can also lead to highly experienced and promoted Panzer crews. With medic units at hand, the rest and recuperation should not take too long.
In enemy cities most enemies will be gunpowder units with city garrison promotions, against which panzers will perform no different than the tank. Even if there are enemy tanks in defense, the game would almost always pick a gunpowder defender. So collateral damage would be the best bet against cities.
This means that the Panzer is no better than normal tanks in the taking of cities, but they are unsurpassed in dominating the surrounding field, something a careful player can utilize in home defense, surrounding a target city to cut of replacements and guerilla-type pillaging raids. They will also provide good stack defense due to bonus versus this era's prime mobile attacker. To avoid Panzers being left alone and unprotected a careful player can use road network in home territory, and counterattack in 1-move terrain only when in enemy lands. Mobility and morale can also be used to get the unit safely home after counterattack.
One final point about the Panzers is that they can still perform when pitted against modern armor. Not quite cost-effective, but a lot better than normal tanks. This could save a situation when the player needs a little more time, either to gain a source of aluminum or research the tech to build modern armor, or to build a last spaceship part.
