The Turks are, IMHO, the most diverse faction in the game. That breeds replayability.

Some of the factions are fun at first, but become tedious and repetitive because of a smaller range of different units. Byzantium springs to mind here. To a similar extent, so too do Catholic nations. It's the same fuedal/chivalric sergeant, feudal/chivalric MAA, fedual/chivalric knight build scheme for almost every one of them.