Though Josh Lyman maintains that the Executive Branch is the most powerful of the three, his teenage doppleganger is correct when he asserts that the Founders, rebelling against the Monarchist form of government, made the Executive Branch the weakest branch. The President and his administration are responsible for executing laws. In the modern world, the United States presidency is a substantial bully pulpit -- and arguably the most powerful position in the world.
* Individuals noted with an asterisk have been mentioned, but have never appeared.