War Chariots. Chariots are already garbage. Unless you happen to start in a flat desert (otherwise you might get forests), they'll get bogged in the terrain and be about as useful as an ordinary archer. The "slowed down by rough terrain" really kills them IMO. Another contender is the Zero, who suffers the same problem as the War Chariot. At least Fighters are much better and I never build them simply because the AI never has that much of an air force, not because they're bad.
At least stuff like the Bowman and the Landsknecht replaces units that you would actually build (Archers, Pikes) and thus have a chance of being used. The Zero and the War Chariot replace units that I already never build because they simply aren't useful. I'm not going out of my way to build them just because they're unique.
