My favourite minor faction is the Crusader States in BKB. OK, they're not that minor as they have three provinces, but they have great units and are surrounded by some pretty tough factions (Eggies, Turks and the Byz). I just love using the "trident" strategy against the Egyptians: making two feint attacks and one real one. Then you usually have the Golden Horde popping up in that neck of the woods.