Which of these men was the first Democrat elected President of the US after the Civil War?

Do you mean who was the first Democrat to be elected President after the Civil War? The Republican party dominated for many years following the Civil War. After Lincoln was assassinated, he was succeeded by Johnson, a member of the War Union Party, who was followed by Grant, Hayes, Garfield, and Arthur, who were all Republicans. Then, in 1884, Grover Cleveland was elected. He was a Democrat, and he took office in 1885.