Initially I'd have said top twenty, but after reading the other responses, I'm not quite as confident. I'd guess somewhere between 15th and 25th at present, maybe. Of course, his second term, if he gets one, could change the equation completely.
He definitely won't be the Roosevelt to Trump's Hoover or the Lincoln to Trump's Buchanan so much as he would be the Ford to Trump's Nixon.
At least top 20. History will be much kinder to him than we are currently.
This. I think he'll be remembered similarly to Jimmy Carter.
This, except no serious presidential historian puts Jimmy Carter in the top 20.