The Casse are a lot of fun. You get to unite Caledonia, move on to Hibernia, spend a decade or two strengthening your economy and filling your coffers before building a massive army to invade the mainland with. I suppose most of my campaign are like this though, whether I am uniting Iberia, Germania, Dacia, or Greece before launching an invasion of a neigboring faction or region. As for the chariots, I agree that they are awful--replace them with some champions and you are good to go. I use Calawre as early bodyguards and Rycalawre as late bodyguards. So no more dissing the Britons.

As for which factions I haven't played, my answer would be most. I have them ranked in priorities and at the moment they are all tied for first place.