I would take Casse any day: their relative isolation protects them, their lands are rich with trade and mining and they have access to a very diverse and powerful roster of Celtic and Goidilic units. Besides, their artwork is superb.
Arverni are a second since I prefer Arjos to Cingetos, but elsewhere they and the Aedui face big challenges with mean resources. Their initial roster also seems to lack in diversity: for some reason I can't even recruit Gaeroas with a high level MIC, as far as I played with them. They also have to fight the Romani and Sweboz after spending their troops and money fighting themselves, which means a big disadvantage and a constant struggle to survive.
Basically, you can choose between the three (Casse, Arverni, Aedui) and tell the reason of your preference.
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