I have to say my favourite faction just down to how challenging and intersting campaign i am currently on with is Imperium Constatne. You have all the roman troops but have to plan much more economy wise. Everything that isn't needed has to be got rid of e.g. i destroyed all churches to enable me to build an army to defend myself from WRE in southern France. At the start constantly on the brink of being anihalted (Burgundi Horde especially) only lost one city to them and took out 12,000 of there army. There location isn't the best considering how exposed they are but it makes it much more interesting i currently have 18 provinces in Nothern Italy, France and Spain, currently eridcating the romans from Spain and the Visgoths from Southern Italy.
What are other peoples favourite faction + why?