If you want to advocate for any matter, what is the best social media with the highest reach to do that?

You want to frame your issue in a way that makes the right wing hate 'o spehere go absolutely insane. They will broadcast your cause far and wide.

I'd say Twitch.

Nothing that requires your real name or data about you imho.

What are your goals? Internet advocacy frequently suffers from poorly defined goals that lead to undesirable patterns of behavior.