You can make an argument for any of them, but I'm going to go with the Presidency or executive branch. Here's why:
Congress has a lot of power. In fact, it has all the power to make laws. So you would think they have the most power.
But in our current system, with the power of the media and all, the President has the most power. He is the one person that just about everyone in the country recognizes. When he talks, the TV stations cover his speeches and pay attention to what he says.
This means that the President, more than anyone else, can decide what issues are going to be on the agenda. In addition, he has more of an ability than anyone else to get people to pay attention and listen to his point of view.