The amendment that has had the greatest impact on increasing civil rights is the Fourteenth Amendment.
Ratified in 1868, the Fourteenth Amendment contains several crucial provisions that protect individual rights, including the Equal Protection Clause, which prohibits states from denying any person equal protection under the law. This amendment has been foundational in numerous landmark Supreme Court cases addressing civil rights issues, including racial equality, gender equality, and the rights of various minority groups. Its broad language has been used to challenge discriminatory laws and practices, making it central to the advancement of civil rights in the United States.
The other amendments listed (Twelfth, Twenty-First, and Twenty-Seventh) do not have the same significant impact on civil rights as the Fourteenth Amendment.