This is intended to be a collection of public opinion polls, surveys of scholars, and other resources which provide rankings of U.S. presidents.
This is a collection of surveys, beginning with a 1998 ABC News poll, provided by PollingReport.com.
2010 results, with more description and analysis than the above.
C-SPAN survey in which 65 presidential historians rate U.S. presidents on ten "attributes of leadership."
This very useful article from Wikipedia is more detailed than the above PollingReport.com collection, but is unsigned and not a stable resource.