One paper will be the most cited. One paper will be the earliest.
Unless they are the same there is not _one_ answer, what you get is an efficient set (the earliest paper with at least X citations / the most cited paper as of D).
And of course there is the problem of making the list of relevant papers in the first place.
Step 1 - Rank papers by number of citations
Step 2 - Which of the most cited papers is the earliest?