How do I find the most cited authors given one or more MESH terms? I would like to be able to give some academic database (like pubmed) a list of MESH terms (MESH: Medical Subject Headings created by the National Library of Medicine), then select papers by authors who have received the most citations. Either in total (all work) or within the scope of the papers I pulled up using the given MESH terms.

Is this possible?
You could use Europe PMC database to search for publications by keyword of choice using (KW:"XXX") syntax. Then sort the results by times cited and look at the authors of the top publications.
Here is an example for most cited articles by keyword "myalgia": https://europepmc.org/search?query=%28KW:%22myalgia%22%29&amp;page=1&amp;sortby=Times%2BCited%2BDESC. In this case, keyword includes MESH terms and keywords supplied by publishers.

If you wanted to do that programmatically, for example to collate citation numbers for individual authors, you could do that via the Articles API. Here is the RESTful URL for the same query: https://www.ebi.ac.uk/europepmc/webservices/rest/search?query=KW:myalgia%20sort_cited:y.

Disclaimer: I work for Europe PMC.