I recommend Baktria, without doubt. Not only is it one of the most complete factions available to you, they have a very nice variety of enemies--their equals in the Seleukids, and their nemesis in the Pahlava. It also provides some massive bloodbaths as you fight Seleukeia as they throw army after good army at you, with the armies getting better the further west you go--I've already had armies made up solely of thorakitai and pezhetairoi coming at me, and if not for the fact that argyraspidai are not in the game yet, I think they would be coming at me too.

And, of course, the Pahlava, whose HA can beat almost any cavalry or foot archers you have. So far that's the furthest I've gone, so I can't comment on the other factions yet.