[{"X":[{"A":"abc","B":123,},{"A":"wer","B":124,},{"A":"fgh","B":124,}]}]

db.collection.aggregate([{"$project":{"X":{$reduce:{input:"$X",initialValue:{},in:{$cond:[{"$gt":["$$this.B","$$value.B"]},"$$this","$$value"]}}}}}])