What the congressional reconstruction greatest accomplishments

Are you talking about the post Civil War? If so, do some research on the Freedman's Bureau. Also you can check out the following site, scroll down to II. Debate Over Reconstruction - read through that part and then scroll down to B. Congressional Reconstruction.
Reconstruction (U.S. history) - Search View - MSN Encarta

Edit: You can read through the following:
Reconstruction
Scroll down to Study Aids and look at Outlines, Lecture Notes and Review Sheets.

Had it run to it's conclusion we may have had the Civil Rights movement much sooner in out history . But the reconstruction regime fell apart before their work was done.