Well,

Just taking on the faction i am playing at the moment - Egypt - I would say Mamluk archers - you can use them early on, they are a fast, good missile unit and once enemy units are weakened, can easily finish them off hand to hand. In terms of adding a winning dynamic to my Saracen armies, these guys make the most impact.

Often as not they can strip out enemy cavalry units, break up infantry formations and take advantageous ground early on.