Arthur Schlesinger Jr. conducted a survey of historians that resulted in a ranking of the presidents in 1948 for Life and another in 1962 that appeared in [The New York Times] (http://query.nytimes.com/gst/abstract.html?res=9E07EEDA123EE036A0575AC2A9619C946391D6CF). Both surveys ranked Abraham Lincoln at #1 followed by George Washington and Franklin D. Roosevelt.

Talks of building a monument to honor Lincoln started almost immediately following his death. Congress passed a budget for the memorial to be constructed in 1867, three years after his death. However the construction of the monument didn't start till 1914 and was completed in 1922.

When Lincoln was young he was well known around Illinois, as he travelled the state as a lawyer. He entertained the circuit with his anecdotes and was well known locally as a fantastic story-teller. He remained unknown for the most part to the rest of the country until he starting a speaking tour in 1860. His Cooper Union speech was monumental and put him in the spot light of the Republican party. After winning the nomination and the election his administration had a rough start. Many believed that Secretary of State, William Seward, was actually controlling the administration and Lincoln was his puppet. But as conflicts started arising for the Union it was Lincoln's political genius that resolved many conflicts. In my opinion, the Gettysburg Address in 1863 was what helped him become labeled as a hero (to the North). The Union had just won a major battle, and Lincoln's speech was a work of art. It was printed in news papers around the country, and became very popular.

TL-DR The Gettysburg Address in 1863 helped label him a hero to the North. Talks of building a monument to honor Lincoln happened almost immediately after his death in 1865.