for cav flavoured slow campaign, i'd like to second the turk/egypt initial (jihad) land grab followed by turtling (sipahi/qupulukus or mamluk archers)

or any iberian faction (jinetes!)

russia is a good choice but you might feel it's too slow/difficult without crusades - the trick is too ramp up the chiv of your governers for the pop growth.

check the faction guides