Gavelkind is the Charlemagne law. Your eldest son will inherit the primary title, and any other kingdom titles will be divided among your sons. You wont get any rebellions, but it makes building a real empire more or less impossible.
Elective works fairly well if you can keep all of your vassals happy, but that's an incredibly tall order especially if you want to expand. Also i can never quite figure out why they vote for one person over another.
Seniority is quite underrated in my opinion, it just has the problem of never really getting a ruler for an extended period of time
I personally stick to primogeniture. Just remember to never ever give anyone in your close family duchy titles ever. If they're distant relatives in your dynasty, then that's ok, but sons, brothers, and uncles are a definite no. The only problem then is the landless sons prestige hit. I get around this by a) marrying your sons to foreign countesses and duchesses, that way they aren't in your court, so you don't get the prestige hit, or b) granting them a county title and then having someone else rule over them as duke. They can't rebel against you because they aren't your vassal.
