[{"detail":[{name:"P1"},{name:"P2"},{name:"P3"},{name:"P4"},{name:"P5"}]},{"detail":[{name:"P1"},{name:"P3"},{name:"P4"},{name:"P5"}]},{"detail":[{name:"P1"},{name:"P2"},{name:"P5"}]},{"detail":[{name:"P1"},{name:"P5"}]},{"detail":[{name:"P5"}]}]

db.collection.aggregate([{$unwind:"$detail"},{$project:{"name":"$detail.name"}},{$group:{_id:"$name",quantity:{$sum:1}}},{$sort:{quantity:-1}},{$limit:1}])