Though deeply engaged in his presidency — battling for the Trans-Pacific Partnership and against ISIS — Barack Obama is also casting an eye beyond Jan. 20, 2017, when his post-presidency begins. We’ve learned that his presidential library will be in Chicago, and he will continue to advocate for young minority men.

Mr. Obama’s slow pivot to his retirement coincides with renewed controversy over how Bill Clinton has conducted his, especially around donors to the Clinton Global Initiative and possible conflicts of interest during Hillary Rodham Clinton’s tenure as secretary of state. Some assessments of Mr. Clinton’s post-presidential career have become extremely critical.

Which of Mr. Obama’s predecessors have been the best ex-presidents and which have been the worst? ...