The JFBCCommentsCount widget is using the fb:comments-count widget provided with the Facebook Comments Plugin, as described
here
.
Any logic to determine which article had the most comments would not be very straightforward and would be done on your part. The fb:comments-count widget just takes a URL, so you would have to find a way to iterate on all of the pages on your site to pass in to that. We've investigated this in the past, but Facebook does not make it easy.
-Melissa