it must be the archer in the early game, for being a good defender, but still a great fighter to wipe out catapults etc.
like many of you guys, I like the dragoon, power full as hell!
then it must be the battleship, for its capability to attack a city, and kill you unit even if the city has coastal defence. And the howitzer, ignores city walls and very, very powerfull!
that's my favourite units

And the sub sucks against the AI, they can of some reason see it 3-4 blocks away, and strike it down whit a fighter or missile!