Both JFK and Lincoln are considered to be great presidents. MLK is considered to have been a great civil rights leader. Using the word "great" to describe these men will seem like an understatement to many. All three were assassinated while still heavily active in their careers, but before historians could reflect an "write history" about these men.
Would the historical work of these men be viewed inn such a favorable light had they not been assassinated as they were? Every man has flaws. However, it's often considered bad taste to speak ill of the dead. So when someone is assassinated as these men were, their strengths an contributions are enhanced while their flaws and weaknesses are minimized.
Also, these men do not have any opportunity to screw up their legacies after the assassinations.
I as somewhat surprised that no one had weighed in on Lincoln yet. There is a large literature in Lincoln historiography that addresses Lincoln the historical figure vs Lincoln the myth. Donald Davis famously stated in the forward of an anthology that each generation of American historians had to first "Get right with Lincoln". A derivative debate deals with what Lincolnian Reconstruction would have developed had he lived and how that would have altered his historical reputation. So from over fifty years of studying Lincoln and Lincoln historiography, here would be my answers.
Would the historical work of these men be viewed inn such a favorable light had they not been assassinated as they were? Every man has flaws. However, it's often considered bad taste to speak ill of the dead. So when someone is assassinated as these men were, their strengths an contributions are enhanced while their flaws and weaknesses are minimized.
We write separate histories of the figures themselves and of the myths. The are related and both are useful, for the history of a myth is the history of an idea, of a set of values.
What popular thought in succeeding generations thinks of Washington, Lincoln, Jefferson, Kennedy, Reagan, or King; no matter how based in the historical record these thoughts are, is important in and of itself.
Lincoln stands alone for a number of reasons. Of all Americans, he was the pre-eminent orator and writer about what fundamentally America was about. His speeches stand up well 150 years after delivery. It is not one speech. The House Divided speech, both Inaugural Addresses, the Cooper Union speech, and the Gettysburg Address all rank in the top dozen of American political rhetoric. Who today remembers the speeches of Henry Clay, John C Calhoun, or Woodrow Wilson, the most renowned speechmakers of their day? In rhetoric, the term "Lincolnian" is reserved for only the pinnacle of such endeavors, and decades can pass with no effort justifying the accolade.
This talent was used for declared purposes, the largest issues of a half century. The roots and branches extend for more than a century in both directions and to virtually every corner of the world. This gives a resonance to Lincoln's thought that ebbs and flows, but never fades. And there lies the importance of the myth.
Washington and FDR confronted crises of comparable dimensions. IMHO Washington while acquitting himself well as the first president, was not a thinker of the level of Lincoln or Jefferson and Madison for that matter. His legacy as a military leader overshadows that of a political leader. FDR confronted great challenges, but I think as time moves him further out of popular experience, his reputation will recede a bit.
Kennedy and King are far more important as symbols of ideas than as actors on the stage of history. Had they not died from assassins' bullets, I am not sure of their places in history.