Its a weighted vote by various economists, politicians, and historians like J. Mikke, Mentzen, Rybinski, Paul, Schiff, Harris, Dawkins, Gortat, Krupa, Storch, Woodward, Krugman etc.
When Harding took office there was a recession and high unemployment. But he left the office with almost 20 percent gdp growth in 1923. Thats just statistics that make him one of the best.
These lists always contain Lincoln while I haven't seen a single one with Warren Harding.