Ok then. I am still fairly new to M2T so I just played vanilla until now, so let's stick with vanilla for the time being.
I have been played with England(finished), Venice(lost) Scotland, Denmark, Turks, HRE; had the most fun with the turks That it offered a differed kind of play. A more creative and abusive, to be honest. So let's go with the eastern factions that have good missile cavalry. Muslim factions, Byzantine, Hungary and such.
I think that having an army consisting of mass missile cavalry offers a powerful tactic. Surrounding the enemy from every side with your horse archers, and use heavy infantry to charge from front if necessary (or let enemy charge). This way the enemy won't be able to use their shields to cover from ranged. This tactic is called as "crescent tactic" by the eastern cultures if I am not mistaken by it's name. Not meaning "false retreat tactic", as some people refer the "crescent tactic" as the "false retreat tactic".