How about Top 5% comments with the restrictions: Top 1 if less than 20 comments, top 10 if more than 200 comments (note: the values can be altered, might also be something like top 5 / 100 comments, not sure what would match this kind of challenge the best).
this could optionally be further restricted/improved by a rule like "only count comments with at least 1 vote" count, or something like "at least x% votes of top 1" (e.g. if top 1 has 50 votes, and at least 20% of the votes are needed, then only the top 5% comments with more than 10 votes are accepted)
this would scale a bit but still require a high enough participation to exclude simple bots.
RE: Can Steemit Discover My Real Identity? - Day 0