THE AMERICAN CIVIL WAR—ONE TIME WHEN THE LOSING SIDE WROTE THE HISTORY. When I was in school in the fifties, we learned a version of American history that favored the Confederacy. Reconstruction was wicked. For that matter, Grant as a general had been a butcher.
I posted here about a 1948 survey of American historians which I think reflected the received view in the fifties. It ranked Grant as the 28th and second worst president (Harding, who also was pro civil rights was ranked as the worst).
There has been some change since 1948 with an increased focus on civil rights. This wikipedia entry has a chart reporting on 19 different rankings of the presidents which have been made between 1948 and 2015. A 2015 poll, administered by the American Political Science Association among political scientists specializing in the American presidency, has moved Grant up so that he is 28th but ahead of 14 presidents (there are a lot more presidents since 1948.) Grant is behind Van Buren. Benjamin Harrison is 29th.