May I suggest the Pahlavân? They may not have as simple a starting position as Baktria, but their form of government is flexible and extends to the benefits of sedentary and nomadic forms; Furthermore, while Baktria has a comparable power in cavalry, the variety of the Pahlavân is unmatched and an equestrian army can almost be contructed to one's deepest wishes. Furthermore, only two factions have the potential to harness the strength and spectra of the Iranian Plataeu, one being Hayasdan in their reformed period, and the other being Pahlavâ; Whereas Hayasdan emphasizes a bit more on infantry, while Pahlavâ focuses on mounted archery, both are very comparable in their late-game potential, except for now, Pahlavân has the upper edge in being able to harness steppen lands. It further means that they have the potential to utterly defeat the Sacae at their own game, and practically make the steppes a frontier of their own.

Another favourable factor is casualty control. When in control of Seleucid, Bactrian, Pontic or Armenian armies, the significant lack of horse-archery, the general has to expect significant casualties, unless in command of battle-hardened veterans or pitted against a clearly inferior foe. On the other hand, Parthian armies are expensive. Luckily, later on this shouldn't be a problem as they ought to have a very "mercantile" campaign game. Furthermore, the Pahlavâ do get one of the most ass-kicking if not the most ass-kicking bodyguard units in their late game, along with an equally powerful recruitable late cataphract unit. The Pahlavân are "poor" on infantry, but not all infantry is poor; The Hyrcanians make excellent shock troops, the Iranian axemen are a well-to-do light auxiliary unit, Persian hoplites are solid when assigned for sentry duty, levy phalangites, some of the finest foot-archers in the mod, late Partho-Hellenic infantry make decent "imitation legions", Babylonian spearmen, Eastern hellenic medium infantry and of course the unbeatable masters of distasters, Iranian archer-spearmen (No, I'm serious... There is no reason not to love those guys, I mean they fire arrows and they can hold a line pretty good until your cavalry can bring the smackdown... What more could you possibly ask for? ). The key is to use infantry only as auxiliaries and/or as garrison personnel; On boot, the Pahlavân get their own flavour of elephantry as well, a representation of elephantry in the Indo-Parthian/Sûrên-Pahlavân/Gondopharid veneer.

For those who cannot live without high performance infantry, artillery and armoured elephants, Baktria fills those voids pretty neatly. But if you want a faction in where you'd like a nearly maxed out cap on available cavalry, including the fine quality horsemanship available in the steppes, while being able to rear a very "Persian" army, the Parthians make a good option. If I really lobby for them, I have even higher thoughts of them