It's harder now since I have played more factions since I was first asked. It would possibly be between Pontus, Parthia and the Scipii. They all are incredibly challenging when placed upon the VH/M settings - the Scipii have to defend against Carthage, Parthia ends up with the whole Eastern world against them while Pontos' feeble unit roster is pretty limited to weaker units such as Pontic light cavalry.