Easily one of the greatest Presidents to ever live. He's almost on the ranking where it's hard to even criticize the man without looking racist or silly. No Presidency is perfect, but lets take a look.
Obviously his presidency is deep within the topic of the Civil War. Even before he took office, the South was planning to secede from the Union. Lincoln did support the Corwin amendment, which would have granted state rights to slavery, which might have prevented war; was pretty fucked up if you ask me.
Alas the Civil War begins. This was a tough time for any President to endure. He is the only commander-in-chief to see his Union split in two. As such, he really expanded his war powers, almost defiantly a bit overboard. Despite his authoritarian-increase, he did remain strong for union efforts. He was always involved in the planning of battles, and he even studied on warfare since he had no real military background to speak of.
Regardless of Lincoln's military tactics, his biggest triumph was the banning of slavery. Luckily done by a Constitutional amendment so slave states couldn't whine about state rights years later. It is a shame that it took so long for slavery to end in America. Yet the South wonders why everyone thinks it's residents are dumb and racist as shit. I'm not proud to be a Southerner myself.
Besides the Civil War, Lincoln was also responsible for a couple other things. Such as giving land cheaply to any citizen, support for the railroads, the first Federal income tax, a national currency, and making Thanksgiving an official national holiday. I'm sure the Turkeys weren't happy though.
It was such a shame Lincoln was assassinated. He was a great man, who yet made many mistakes, but got passed them and put this nation back together. Is he the greatest President in history? Again, that's a tough call for anyone to say. But he's easily in the top ten.