Gauls, Aedui are easier at the start. Those are always fun games. Plus, they probably have the best economy out of all of the barbarian peoples. They get paved roads and stuff. Their elites beat the crap out of just about anything. Crazy naked dudes and heavily armored swordsmen will easily conquer anything in Europe. Their reforms are really easy to get too. Just build a lot of markets and temples, then build up your population growth and expel some big cities, and you can get your best troops pretty quickly.