In theory, the right to freedom of speech—recognized in the 1st Amendment to the US Constitution—is the most important right, because it informs our ability to determine what constitutes a right, what constitutes and important right, and how to rank the relative importance of rights in the first place. However, as a practical matter, I argue that the most important right is actually...
THE 7th AMENDMENT TO THE US CONSTITUTION
This Amendment recognizes your right to a trial by jury. The 7th Amendment states:
In Suits at common law, where the value in controversy shall exceed twenty dollars, the right of trial by jury shall be preserved, and no fact tried by a jury, shall be otherwise re-examined in any Court of the United States, than according to the rules of the common law.
What's so important about the right to a trial by jury?
The ability to present your case to real human beings selected from your community is essential to the protection of all other rights. In fact, there is no other way to protect your rights other than vindication of those rights before a jury.
Let's take a concrete example to see why. The 4th Amendment recognizes your right to be protected against unreasonable searches and seizures. Imagine the police bust down your door in the middle of the night and haul you off the jail. You've done nothing wrong; the police have the wrong address and think you're someone else.
What happens if you try to use your 2nd Amendment right to protect your 4th Amendment right?
You get in a shootout with the cops and eventually die in a hail of bullets.
What happens if you try to use your 1st Amendment right to protect your 4th Amendment right?
You calmly explain that the cops have the wrong person and that you've done nothing wrong. This is precisely what everyone says to the cops when they're being arrested, guilty or not. This is unconvincing to the cops, who are going to arrest you anyway.
The 7th Amendment.
The fact of the matter is, under this kind of circumstance, you're going to jail and can only get vindication of your right to be protected against unreasonable searches as seizures. This is where a jury comes in.
Your ability to explain your case to a neutral and unbiased jury (in other words, you don't have to put up with a jury made up of, for example, the cops who arrested you), and to be justly compensated for the violation of that right, deters the government from violating that right going forward. In this example, a jury trial and the threat of money damages awarded by the jury are the only way to hold the police accountable for violating your 4th Amendment right.