There is no “best extracurricular.”
The best thing to do, from what I’ve read and in my opinion, is to pick one or two things you’re really good at and that you really enjoy and just devote yourself to them.
The only “requirement,” really, is to make sure that there’s some way you can show proof that you excelled in that specific activity. That’d mean participating in competitions if applicable. Other than that, though, if you’re passionate and skilled at it, that’ll lead to more opportunities and also be the most genuine on an application. Additionally, it’ll make it easier to write essays about it bc it’ll be something you actually care about.