Second, the other branches are largely incapable of interfering with Congress. Members of Congress are immune from arrest when they are conducting legislative business. They set their own pay. Each chamber determines its own rules. Each branch also decides who does and who does not get to sit as a member. And no member is allowed to take a job in the executive department while serving in Congress. The Founders were so serious about this that they established a capital city apart from the states, and under the jurisdiction of Congress, to make sure that some local potentate could not use geography to boss the legislature around.
Take these two concepts together and ask yourself: If I get to tell you what to do, but you do not get to tell me what to do, who is actually in charge?
Third, think of the mandate that each branch has under the Constitution. The executive power is the power to enforce the law. The judicial power is the power to resolve controversies arising under the law. The legislative power is the power to make the law itself. By necessity, it comes first.