States who fought for the Confederacy had to apply for readmission to the United States before they were allowed to send new congressional representatives. Not only did they have to re-elect their state governments and congressmen, but they also had to approve of the 13th amendment and reaffrim their loyalty.
Tennessee was the first to seat new representatives in 1868 after they were readmitted in 1866. There were also several other states who came back to congress, but Tennessee should be considered they first due to the date of their re-admittance.
Georgia was the last to send reps in 1870. They had originally been re-admitted early on, but were removed when their state legislature attempted to eject the newly elected African American representatives.