See you posted a few times here, again, here is my answer:
For me, the most important piece of the Constitution is the Supremacy Clause contained in Article VI, para 2. The Supremacy Clause gives the the Federal Constitution supremacy over the States and forbids the States from interfering with the Federal government's lawful exercise of its constitutional power.
For me, this is the most important piece of the Constitution as it structurally defines America's unique and, at times, problematic system of Federalism. From there, the rush is on to define what the Federal government's lawful constitution power is but I believe that it allowed the country to grow into its size and work through the kinks of a series of states with divergent interests coming together as one unified country.
