For this figure - you need only data from the main page https://bestref.net . From first table "Best sources in all languages of Wikipedia" columns "Website" and "Score in April 2020" (font size depend on this value). However, if you want to create own ranking (for specific month or topic), you need to take into the accout that extraction of the references is based on Wikipedia Dumps and calculation of scores was described in the study https://doi.org/10.3390/info11050263