To my mind, the First Amendment is without question the most important in today's world. It's guarantee of freedom of speech and the press guarantees an open expression of honest disagreement, even if the views expressed are unpopular. Society benefits from the free expression of opposing ideas; as stated in Hegel's famous argument, it is the thesis and antithesis which create synthesis.
No less important is the Amendment's protection of people's right to
peaceably assemble , and to petition the government for a redress of grievances.
Unpopular though many of these "assemblies" may be, they also provide for an open exchange of ideas and are the surest guarantee against a government which silences debate.
Finally, the amendments "wall of separation between church and state," as stated by Jefferson not only guarantees each person the right to observe the religious belief of his choice, it also protects one from the imposition of beliefs to which he does not adhere.