[{"key":[{a:-23,b:"b"},{a:-34,b:"d"},{a:-15,b:"c"},{a:-51,b:"v"}]}]

db.collection.aggregate([{$addFields:{winner:{$reduce:{input:"$key",initialValue:{maxValue:{$max:"$key.a"}},in:{$cond:[{$eq:["$$this.a","$$value.maxValue"]},"$$this","$$value"]}}}}}])