It's popular now to say he's one of the worst, but I've lived too long.
I think part of the answer is going to depend on how things work in the future. If the Trumpites continue their dominance of the Republican party, and if they're able to win in future elections, and/or if some of Trump's actions/policies turn out well, it's possible his reputation will be higher than we think now.
I point to the instances of Grant, Truman, and Nixon, all of whom were disdained when they left office but whose reputations have revived since. Elizabeth Drew had a Post Opinion piece on Nixon's path back. I'd forgotten but she says he'd regained respect by 1980.
Grant used to be thought a terrible president, mostly because of corruption. But these days his handling of the South during Reconstruction has gained him a lot of respect, particularly from historians and the recent biography by Chernow has cemented it.
Truman when he left office was very unpopular, regarded as someone who had diminished the presidency by his demeanor, his fight with MacArthur, his Korean war "police action". IIRC Nixon called it an administration of crime, corruption, and communism. But as the Korean war faded and his civil rights policies came to the fore his reputation has gained.
Trump could experience similar gains, but I hope not during my lifetime.
[Updated--Post has a piece on changes in reputation here.