It is merely a theory that explains how free enterprise works. Banks for example, a very important part of the economy, are left out. Some socities do not accept the circular flow model theory. These socities, therefore, choose another economic system other than capitalism. Socialists and communists see the role of the government in a different light than capitalists do.
The United States Constitution states that the Vice President of the United States is the President of the Senate, despite not being a senator, and that the Senate must choose a president pro tempore.
Since the question is most powerful PERSON, rather than most powerful Senator. According to the Constitution the President of the Senate, ex officio, also serving as Vice President, is the highest ranking officer of the Senate. Joe Biden. 2nd most powerful person, most powerful Senator, is President pro tempore, Robert Byrd. Also 3rd in line, behind VP and Speaker of the House, for the Presidency.