This question obviously requires an answer that is opinion-based. While it is tempting to say that Lincoln's most important contribution to the nation was the Emancipation Proclamation, I think his work in preserving the Union is his most important accomplishment. The Emancipation Proclamation actually only freed the slaves in the Confederate states and did not affect the border states or the states that remained in the Union. The Thirteen, Fourteenth, and Fifteenth Amendments, all of which were passed after Lincoln's death, were the real laws that granted African-Americans their legal freedom. Lincoln, who felt that the secession of the Southern states was illegal, rallied the Northern states and the Democratic party behind the epic fight that was the American Civil War. His leadership during this conflict is recognized as an important factor in the Union's victory. Lincoln's tireless effort to preserve the United States was his most important accomplishment.