Elective is always quite risky, although it's less unpredictable if you have a large realm. Primogeniture is the safest, easiest option which also allows you to plan ahead well. Seniority is the weakest option in my opinion, although it has a few advantages: you'll never have to suffer through a regency and when your dynasty has expanded all across the continent, Seniority often allows you to eventually unite the dynasty holdings under a single ruler. Its disadvantages are more numerous, though: always the short reign penalty crippling you, very hard to make sure your heir is competent, and extremely hard to switch out of (because you don't have enough time to stabilise your realm in order to fulfill the requirements for succession law switching). In short: if you have no luck with Elective, the best way to go is Primogeniture. Indeed, I almost always go with that.
(My favorite is the Turkish succession, though. It's like elective, with only you as the elector and only your sons as the candidates. However, it's only for Muslims, unfortunately. If you want to try that out, I recommend a minor in Northwest Africa at first.)