What is the highest authority in the US?

Answer 1

The ultimate authority in the United States is the citizenry, as that is where all legal sovereignty lies. In terms of the existing government, the Constitution is considered the highest authority, as it defines the structure of the government and the powers of all sections thereof.

It even explicitly states in the Constitution (the so-called Supremacy Clause):

This Constitution, and the Laws of the United States which shall be made in pursuance thereof; [...] shall be the supreme law of the land.

- Article VI, Clause 2, Constitution of the United States

Thus no judge, President, Congress, or any other authority can legally pass a law, make a ruling, or perform an action that contravenes the Constitution.

It could thus be argued that the Supreme Court is therefore the highest authority in the government, as the Supreme Court is charged with interpreting the Constitution, and it alone ultimately decides whether an action or law is Constitutional.