The amendment that has had the greatest impact on increasing civil rights is the Fourteenth Amendment. Ratified in 1868, it contains several clauses that guarantee citizenship rights and equal protection under the law, significantly shaping civil rights in the United States. The Fourteenth Amendment has served as a foundation for various landmark Supreme Court cases related to civil rights, including issues of segregation, discrimination, and voting rights.
The other amendments listed, such as the Twenty-Seventh and Twelfth, do not have the same direct impact on civil rights. The Twenty-First Amendment pertains to the repeal of Prohibition, and the Twelfth deals with the electoral process for the President and Vice President, neither of which fundamentally focus on civil rights issues.