Database
[{_id:{userId:"5df344a1372f345308dac12a",pool:"foo"},correctBets:9},{_id:{userId:"5dfb2db245011a390fec4f7d",pool:"bar"},correctBets:6},{_id:{userId:"5df344a1372f345308dac12a",pool:"bar"},correctBets:6},{_id:{userId:"5dfb2db245011a390fec4f7d",pool:"foo"},correctBets:11}]
Query
db.collection.aggregate([{$group:{_id:{pool:"$_id.pool"},data:{$push:"$$ROOT"},correctBets:{$max:"$correctBets"}}},{$unwind:"$data"},{$match:{$expr:{$eq:["$correctBets","$data.correctBets"]}}},{$replaceRoot:{newRoot:"$data"}}])