Abraham Lincoln is mostly regarded as the greatest president for his leadership during the Civil War and the abolition of slavery. His main contender is Franklin D. Roosevelt, for leading the country out of the Great Depression and during World War II. Some surveys award the top spot to Founding Father and first president George Washington for setting several enduring and important precedents for the office of the presidency, including the peaceful transition of power.
https://en.wikipedia.org/wiki/Historical_rankings_of_presidents_of_the_United_States