Who is the most powerful person in America today?

a) The President 

b) The chairman of the Federal Reserve
N.B. the chairman of the Federal Reserve can wreck an administration by a few well chosen words or by wrecking the economy through policy. 
He/She can help decide elections - Bernanke helped Obama by announcing QE3 just before the election (rather than waiting until just after).
Romney said he was going to replace Bernanke before the election (silly boy). 

c) Keith Alexander 
Knowledge is power. 

Just like J. Edgar Hoover held power over every significant person in America through his surveillance dossiers. 
No politician dared to say anything against Hoover.