In addition to what the previous editor stated regarding the executive branch and our system's balance of power, it would be illogical to argue that the executive branch is the most powerful part of our government simply because it is the only part of the three main branches that has a term limit. While senators and congressmen can be reelected for decades and while Supreme Court justices are appointed for life (or for whenever they choose to retire), the President must submit to the four-year term limit. Even though he may run again for another four years, he must constantly keep in the forefront of his mind that the role is his for a maximum of 8 years.
Our system of government was set up to have three branches of government, and so no branch would have more power than the other two, the framers of the Constitution set up a system of checks and balances. Under this system, each branch has the ability to stop the other two branches from gaining too much power. For example, one way the executive branch (the president) is able to stop the legislative branch (Congress) is through the presidential veto. The president can veto any bill that has been passed by Congress and stop that bill from becoming law. The judicial branch (Supreme Court/court system) has the ability to check the president and Congress through judicial review. With this power, the Supreme Court can declare a law that has been passed by Congress and signed by the president unconstitutional. Congress has checks on the Supreme Court and the president as well. To answer your question then, no one branch of our government is more powerful than the other, so the executive branch isn't the most powerful branch of government.