What Supreme Court case did the most to strengthen the power of the federal government?

The Marshall Court's decisions strengthened the federal government by clarifying the relationship between the federal and state governments, and supporting Congress' right to regulate certain activities.

This was necessary because the Constitution assigned specific powers to the United States that the federal government declined to exercise in its early years, allowing states to fill the void with their own laws and policies. As the population grew and technology developed to allow more interaction between the states, they soon fell into conflict with each other over regulatory issues. The Constitution provided a remedy for this predicted conflict in the form of the Interstate Commerce Clause, which allowed Congress to regulate interaction between the states. The states naturally resisted ceding their traditional control over what they considered states' rights matters, leading the Court to invoke the Supremacy Clause and assert the federal government's dominance over the states.