[{"_id":"1","array":[{"user":"1","A":2,"B":0},{"user":"2","A":3,"B":1},{"user":"3","A":0,"B":5}]},{"_id":"2","array":[{"user":"4","A":1,"B":1},{"user":"5","A":2,"B":2}]}]

db.collection.aggregate([{"$addFields":{"total":{"$add":[{"$sum":"$array.A"},{"$sum":"$array.B"}]}}},{"$sort":{"total":-1}},{"$limit":1},{"$project":{"total":0}}])