Voted for Quechua and Immortal/War Chariot. Some other units are excellent too, but the key thing about these, particularly the Qs, is that they come very early, and are cheap. This game is all about getting off to a good start, get the snowball rolling, and these units do this very well. Of course, for the Unique Chariots, you need access to horses, and that's by no means a sure thing.