Definitely the Turks. i think they're the most intriguing of the muslim factions, with all their really interesting and unique troops. The janissaries are too cool, plus the Sipahi of the Porte. Basically they're the only muslim faction with troops to match the catholic factions in late game. when the catholics get chivalric knights, chiv men-at-arms, gothic sergeants etc., only the turks can match them technologically.

plus i just love their history. from basically nowhere they stormed onto the world stage and carved a great empire that lasted more than 500 years. at one point in history, they were the scourge of the west. even today they're the most progressive and secular of muslim nations.