The most important power that Congress has must surely be the power to make laws. This is the most fundamental power in any system of government. Nothing can be more important than this power.
Congress has many powers. It has the power to impeach and try the President and other officials. It has the power to conduct hearings. But the importance of these powers pales when compared to Congress’s power to make laws. Because this seems like an obvious answer, I wonder if you are asking about the specific types of laws that Congress is allowed to make. If that is what you are asking, the most important power of Congress is the power to tax and to spend.
There are many other powers that are important. The power to regulate commerce between the various states is one. This power has been used to make regulations in a variety of areas, including such areas as anti-discrimination. However, the power to tax and spend is more important yet. Taxation, of course, is a way of taking money from people. That is clearly an important power. The power to spend money is also extremely important since it gives Congress the power to do everything from funding the military to building the interstate highway system. For these reasons, making law is Congress’s most important power and laws on taxation and spending are the most important laws.