Instead of "RANKING" members - simply provide a list of those with full collections in alphabetical order of username.
There is no
ranking of the full collections, the entries are just just numbered. You are assigning a meaning to a number where there is none and then, apparently, treating it as a competition for no readily discernable reason.
The list has to have some order, the order used is no more meaningful and no less meaningful than alphabetical order (or a purely random order). You would still be moving down the list as people with names that came earlier in the alphabet completed their collection (and you would be permanantly near the end). I would prefer alphabetical order but that is mainly because it makes finding a name easier in the app where there is no in page "find string" function as there is if you use the web site.