Checks and Balances, Separation of Powers, Federalism and Limited Government

Which of the four principles of the Constitution is the most important?

Personally I believe that separation of powers is the most important principle of the Constitution.
Without it our government would be the most inefficiency and completely useless to the general masses.
Veto is the power the president has to completely deny a bill that has passed through the senate and been sent to him.
2/3 is the amount of the Senate needed to pass a bill. This isn't everyone ( that'd be crazy impossible to get everyone to agree on the same thing) so the 2/1 allows there to be an enough of a majority.
Review is the power that the Judicial branch has if they believe that a bill that has already passed is "unconstitutional". They can completely reset the bill and require it to start over.

The Declaration of Independence and The Preamble of the Constitution

The Declaration of Independence is exactly what it says it is, but the Constitution is not. In the Declaration of Independence the United States, states that the thirteen colonies is now separated from Great Britain. And in The Preamble to the Constitution the United States claims the rights and laws its own people will abide by in the newly free land. Although they are both, in a lot of ways, just lists, they are focused on different aspects of the freedom. One is the reasoning for why they're separating themselves and the other is that now that they're free they'll need some ground rules so it doesn't become an anarchy.