In many ways Lincoln...
This isn't to say that he was the only "divisive" President, or that he was wrong to take the stances he did. In fact, a real decision with regard to slavery was LONG overdo, after nearly a hundred years of independence with slavery larger than it was at the revolution and the American people beginning to kill each other over the issue... ANY man that gets elected on a platform to restrict slavery, as Lincoln did, would be bound to face major trouble... and trouble that would HAVE to be faced eventually.
The 1860 election bringing Lincoln to office remains the only election that Americans have gone to war with each other over... if only to move toward the better angels of our nature... and become the kind of nation that cannot imagine HOW the Civil War was possible...
Reagan isn't particularly liked by Democrats today, and a fair number of them in 80s probably didn't like him then, either, but the Democrat lead states didn't secede from the Union over Reagan. The same cannot be said for southern Democrats (with a few exceptions) in 1860-1861.