I'd say try the Lusotannan. I've never played them but they sound good. They get the best infantry in the game (Dosidataskeli) and the best horse in the east (Iberi Lanceari). Though, their armies are more about skirmishing than fighting I think.

Another choice would be the Getai, they have some good units, or the Sweboz because they can train 4exp units
https://forums.totalwar.org/vb/showthread.php?t=121193
https://forums.totalwar.org/vb/showthread.php?t=120688