[{title:"1",US:{data:{lastReportDate:"2021-09-09"}},GB:{data:{lastReportDate:"2021-09-04"}}},{title:"2",US:{data:{lastReportDate:"2021-09-07"}}},{title:"3",US:{data:null}},{title:"4",US:{data:null},GB:{data:{lastReportDate:"2021-09-08"}},NZ:{data:{lastReportDate:"2021-09-04"}}},{title:"5",GB:{data:null},NZ:{data:{lastReportDate:"2021-09-06"}}}]

db.collection.aggregate([{$project:{doc:{$objectToArray:"$$ROOT"},title:"$title"}},{$unwind:"$doc"},{$match:{"doc.k":{$nin:["_id","title"]}}},{$group:{_id:"$doc.k",maxDate:{$max:"$doc.v.data.lastReportDate"},titles:{$push:{date:"$doc.v.data.lastReportDate",title:"$title"}}}},{$project:{_id:0,country:"$_id",maxTitles:{$filter:{input:"$titles",cond:{$eq:["$$this.date","$maxDate"]}}}}}])