I imagine it could be based on which SO site has the lowest median and the lowest mean of user points, correcting for the number of members and the age of site (since newer sites will naturally have lower averages).
That's only an idea. Does SO even do such comparisons? If so, how does SO determines which sites are the lowest performers? What are the metrics?
PS: for the sake of simplicity, let's define performance as a lot of people with a high average of points. A lot newbies entering also means the site is performing well, but if those newbies don't participate and help raise the average user point count the site isn't performing (in sense defined here) that well. So in the end, high point avg seems make sense.
UPDATE: I'm referring to a "site rankings" context. Not necessarily to find out which are best, but to identify and help the under performers. It enables trying to improve them or take them out of circulation if nothing helps.