The Aedui are just the right size and have an interesting starting setup and family, with the possibility for some new traits. They were the first faction I tested with, and I've had a lot of fun with them.

I like the Greeks for similar reasons (except being the first I played, lol).

Swêbōz and Baktria have some challenging battles right off the bat. If you like a lot of fighting, you should enjoy them.