Some of the Best Factions are French, Ethiopian, and Zulu
The Worst are Americans(still think so), Paraguayans, and Bulgarians(never thought I would say it)
The best factions are pretty well known....France has advanced vills and Very great Cavalry, Ethiopia has literally mini-forts as "buildings" and good military, and Zulu are just scary with the fact that they could easily wipe you out 2nd age.
For worst, I find that I can never get on the offensive with Americans, I honestly don't understand how to function with them and the opponent never gives enough time to learn how to.
Paraguayans are the definition of weak. Forget Russians and even the Americans. No, this is probably the most weakest, smallest attack and hp you will find, and the civ also takes up an intense amount of space.
Bulgaria was my original favorite civ. I really liked there infinite cards and there troops are beast. The problem is before you get them, you need to survive 2 ages and then they still train very slowly. I really hope they get either some weak troops or better military spawning system, but at this point they are bad, with the simple fact that for almost whole game they are exposed.