[{_id:1,links:{"google.com":3,"facebook.com":4,"whatsapp.com":6}},{_id:2,links:{"google.com":4,"facebook.com":6,"yahoo.com":8}},{_id:3,links:{"mail.com":3,"twitter.com":4,"whatsapp.com":5}}]

db.collection.aggregate([{$addFields:{links:{$objectToArray:"$links"}}},{$unwind:"$links"},{$group:{_id:"$links.k",count:{$sum:"$links.v"}}},{$sort:{count:-1}},{$limit:1}])