[Izulde]

Depends on what level you're looking for, what type of game you want to play, etc. Germany can be fun if you have the patience to wait until for the rebellions to start. Spain is a tough situation similar to Nubia. Ireland is a little overdone, but there's stuff to do there. Anywhere in Central/Eastern Europe brings the opportunity for pagan bashing, as does Scandinavia.

If you're going for a quadfecta though, I'd avoid one of the strong kingdoms or things will be very boring by Vicky at the very latest, because you'll own almost the entire world.