Take Germania. You get everything you expect from Barbarians. You start with an economy that doesn't even feed your existing population; you have cities that grow increadible slow. You have easy picups all around; You have immidiatelly two enemies (Gaul and Brittany) to start with and a long term enemy in Rome.

You have cool unit s even at the start. Spearbands beat everything, screaming women are an early flanking element, you get barb cav soon: Later you can upgrade to Gothic cav, which is really good. You have some special units like naked fanatics and berserker.

So enjoy it.

If you are more cavary focused take Scythia. They are poor as well and you have nothing but horses.