Which of the six basic principles of the Constitution is the most important?
Indeed, limited government is an important principle in the Constitution, but I would argue that if we consider the context in which the Constitution was written and ultimately ratified, federalism is perhaps the most important of its principles. This is because the Philadelphia Convention was called by those who wished to establish a central government that was much stronger than that under the Articles of Confederation. The debates at the Convention reflect significant, even bitter, differences over the extent of the powers of this new national government, but most agreed that it needed to be strengthened. Yet many sought to preserve at least some of the powers of the state governments, which were essentially sovereign under the Articles. The compromise they developed was a system that has become known as federalism. Under this system, the national government established by the Constitution was supreme (the Constitution itself was the "supreme law of the land") and invested with significant new powers and the ability to accrue more through the expansive interpretation favored by some of the founding generation. Indeed, some delegates, including James Madison, wanted a federal "veto" power over state laws. Yet the states retained some powers, a concession to those who feared the federal government would not be able to rule such a large country without becoming tyrannical. This system is the foundational principle of the Constitution, and the extent of state and federal powers within it have been a major source of political debate and conflict ever since.
The most important of the six basic principles of the Constitution is the principle of limited government. The other five principles of the Constitution are largely meant to ensure that government remains limited.
Limited government is the idea that the government cannot simply do whatever it wants. There are limitations that are placed on what the government can do so that it cannot take away the rights of the people. For example, the First Amendment explicitly prevents the government from depriving us of our freedom of speech. There are many other areas in which the government is not allowed to interfere. The point of this is to prevent the government from tyrannizing us.
All of the other basic principles of the Constitution are arguably meant to ensure that government will not be able to tyrannize us. We have popular sovereignty, which allows us to control the government so that it will not abuse us. We have separated the powers of government between the various branches and we have given them checks and balances over one another, including the power of judicial review. In addition, we have divided powers between the federal government and the state governments. All of these things are devices to make sure that no one part of the government becomes too powerful. A part of government that was too powerful could tyrannize us.
In short, the other five principles of the Constitution are all meant to strengthen the principle of limited government.