LightSpectra
me autem minui
Without question, the hardest faction in the game (in SS) is the Kwarezmians. Your entire game is essentially built around getting ready for the Mongol Horde, and only by the grace of God will you not be gangraped by the Fatimids and Turks before then.
I've never tried the Kwarezmians, but the AI usually has them as the largest faction in my games. Maybe it's because I'm crusading the Fatimids into submission.