By interest it would be Iroquois, then Persians, then Mughal. All others are probably very similar to some existing faction, except perhaps the Crimean Khanate, but the Persians would be similar enough.

However I assume "playable" means playable at the beginning, and all factions can be unlocked once you beat them. So I chose Mughal. For this reason: the Iroquois can be easily unlocked by playing as Americans, or British or French. The Persians can be easily unlocked by playing Russia or perhaps Uthman/Osman Empire. The Mughal are the only very far away one, and it would be more convenient that way, else you'd have to unlock it by fighting till the ends of "the world".