Who is the de facto most powerful person in the United States? (read OP)

[MarkD]

I'd say the 9 justices of SCOTUS, but if I had to be more specific, Anthony Kennedy since he is the swing vote on a lot of important cases (Obergefell v. Hodges)

Yup. He writes so many of the most important Court decisions: Bush v. Gore (it isn't in the official record that he wrote it, but Alan Dershowitz makes a convincing argument that Kennedy wrote it), Windsor and Obergefell, and Citizens United v. FEC.