I have to go with LBJ. His presidency was fatally marred by the Vietnam war, but, if you can set aside a no win situation for everyone involved and poor decision making all around for that, Johnson did one thing that very few presidents have done. He made a hot button topic (Civil Rights) his platform and did everything he needed to make what he could happen - despite the political fallout for himself personally and for his party. He did what was right because it was the right thing to do.
Carter tried, but didn't have the political clout to pull it off. Lincoln did it, but wrong century for the question.
I don't believe that you can judge a president's accomplishments solely by how many people died or didn't die during his presidency. By that criteria, Lincoln would be the lowest on any presidential poll by the fact that more people died during his presidency than any other and to top that off, they were almost exclusively American lives!
Taking that further and blaming WWII presidents for the atrocities committed by Hitler is unfair to any one person's legacy. No one person, not even a president, could have stopped Hitler from doing what he did.