According to the SCOTUS, it’s the Tenth - "The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people. " It’s been referred to as a “truism” by the court, and no case has applied it favorably (i.e. ruled a law unconstitutional because it violated the Tenth).
One can argue that the existence of the Third Amd. has prevented such to take place, so it’s been remarkably useful.
By the same token, the Ninth, “The enumeration in the Constitution, of certain rights, shall not be construed to deny or disparage others retained by the people” is pretty much useless, given the broad interpretation given the Commerce Clause (Art. I, Sec. 8, clause 3 - “Congress shall have the power…To regulate Commerce with foreign Nations, and among the several States, and with the Indian Tribes…”), by the courts. However, arguably one case of recent vintage (U.S. v. Lopez) has reined in the abuse of the Commerce Clause, and may have breathed some hint of life into the Ninth. However, the court has not revisited the Commerce clause since, to the consternation of legal scholars across the nation.