Panzer. Not only a very effective weapon in an age where every city has 6 or more defenders, but also you can go really deep into enemy territory and so disrupt supply lines, resources and stuff like that. And it looks really cool too!
Second choice would be Berserker. Attack of 6 early in the game and need no resources to build! Can attack of a ship, so no need to have them deployed next to a city first. And if the first city is taken then you can in the same turn take another city if you have enough Berserkers in your ship left. Unlike with normal troops, because you have to redeploy them and all.