I don't know how you can rank who the most divisive President in history was, but Obama's divisiveness was calculated, effective, and yet subtle. It wasn't the policies that were passed through nearly as much as it was rhetoric and actions. He constantly berated anybody right of center, not through crass twitter posts but through comments about things like clinging to guns and bibles, you didn't build that, Fox news rants, refusing to call on any journalists from non-left media, chiming in on every single social justice cause, comments about elections having consequences and him ruling for the people that voted for him. But is he any worse than most other Presidents? That I don't know.
Much of the divisiveness is a natural progression from the first Gulf War, through the Clinton scandals, then the attacks against W about him being the village idiot and the myth that was "W lied and people died," and then the rapid evolutions of the last two Presidencies. I think it's more of a cultural war going on irrespective of the person in the White House. We can look to them as the cause, but I think that they are more of a focus or a symptom than the actual cause of the division.