Echoing /u/VM1138, Lincoln was an incredibly pragmatic president. He said at one point that if not freeing the slaves meant saving the Union then he would do that. His presidency is so unique in that he was able to put the Union back together during its darkest crisis. As Eric Foner points out in The Fiery Trial Abraham Lincoln's views on slavery evolved over time. We aren't sure what they would have become if he hadn't been assassinated.

Lincoln's views toward settling freed blacks and slaves in Africa is not overlooked by historians. But we have to keep in mind that this was merely an opinion of his, and as such just one aspect of his overall philosophy. Truth be told, Lincoln was also a very practical man. When president, any consideration he gave to colonization could be seen to have been his way of considering every option when faced with the freeing of the slaves. Such an act transformed the American landscape and he had no real precedent to build from when considering racial integration.

Lincoln freed the slaves. He did not necessarily believe in the equality of the races, but he did emancipate the slaves. So even if he supported colonization his reputation as a saint to freedom isn't misplaced.

Sometimes it can be difficult to separate Lincoln's privately held views from his constant pragmatism, but colonization was not his reason for freeing the slaves.

As to the question stated in your thread title, he is considered great not only because he freed the slaves, but because he presented himself as a wise and reasoned leader and played a significant role in creating a more centralized American government that is more familiar to us today than antebellum America would have been. And Americans have a tendency to elevate wartime leaders.