Here are the three Reconstruction amendments:
13th amendment- This outlaws slavery in the US.
14th amendment- This states that if you are born on American soil, then you are considered a US citizen. This also establishes the Equal Protection Clause, meaning that individuals must be treated equally under the law.
15th amendment- This gives African-American men the right to vote.
A case can be made for all of them. However, outlawing slavery would probably have the most significant impact. This got rid of an institution that was part of American history for over a century. Along with this, it completely changes the way the Southern economy will work.