I have to say I love every faction almost equally. They are all so diverse and unique, it is impossible to choose just one. When I first started playing, unfortunately I made assumptions about how fun it would be to play various factions, so I never bothered with Hayasdan, Quarthadast, Aedui, Lusitania, Pahlava, Seleukids, etc and I missed out on a lot. I would stick to factions I knew something about, like the Romans or a mainland greek faction, but half the fun of EB is learning about history, and there is no better way than to try a new faction. You learn about the types of governments they had, important monuments, their culture, their rulers, their military. It's quite fantastic.