The Bosporian Kingdom. I've "played" them in EB1 by migrating the KH to Pantikapaion in year 1 using console codes and bribery whilst abandoning their starting provinces. They were a hell of a lot of fun.

The hoplites and (horse) archers military was refreshingly different from the other miltaries. Their position and mix of enemies is also interesting -- you can end up fighting Sauromata, Getae, Hai, Sweboz, and Pontos/Seleucia if you cross the water.

For people who want to play EB2, rather than <grin>pontificating on the forums</grin> about map balancing or historical importance, this should be a really good faction.