I'll cast my vote for either Getai or Pontos.

Getai - Falxmen will waste everything, literally! The Getai also have access to some really nice horse archers, making their campaigns even funnier.

Pontos - Easily one of the toughest factions to play, perhaps even THE toughest. It all comes down to many years you can stay allied to AS while building up huge armies of your own for later invasions of Anatolia. The big issue though, trying to build an army with constant debt...