[{"name":"A","upvotes":["a","b"],"downvotes":["a","b","c"]},{"name":"B","upvotes":["a"],"downvotes":["a","b","c"]}]

db.collection.aggregate([{"$addFields":{"diff":{"$subtract":[{"$size":"$upvotes"},{"$size":"$downvotes"}]}}},{"$sort":{"diff":-1}},{"$unset":["diff"]},{"$limit":1}])