I prefer Agnatic-Gavelkind succession laws. I don't know why but it always works for me, I've been playing for 200 years and have a dozen kingdoms to my name (Rus, Sweden, Norway, Lithuania, Bulgaria, Georgia, Hungary, Poland, many more), realm size of 400+, thats a whole third of the world. Succession laws are different for each kingdom, its important to guard your titles. Usually takes me a whole two hours before I allow a day to pass, because its difficult holding all these together now. One day you rule the world, the next your a lowly count. And its always a constant war with the vassals, they will never be wholy satisfied. I just wish I could do away with the dukes and hundreds of counts. Once a year a count or duchy tries to become independent, but 200,000 Russian soldiers puts them back in their place.