How can I compare values fromdifferent records

Hi Ketil_F, check this:

Output

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WclTSUXJJTVYwNAIyFCwNlGJ1kASNQYIWpqiCpmBBCxRBIzOwdhNUQXOwSmOl2FgA", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Name = _t, Date = _t, Score = _t]),
    ChangedType = Table.TransformColumnTypes(Source,{{"Score", type number}}),
    GroupedRows = Table.Group(ChangedType, {"Name"}, {{"T", each 
        [ a = List.Buffer([Score]),
          b = List.Generate(
                    ()=> [ x = 0, y = a{x}, max = y, z = "X" ],
                    each [x] < List.Count(a),
                    each [ x = [x]+1, y = a{x}, max = List.Max({[max], y}), z = if y > [max] then "X" else null ],
                    each [z] ),
          c = Table.FromColumns(Table.ToColumns(_) & {b}, Value.Type(Table.FirstN(_, 0) & #table(type table[New best score=text], {})))
        ][c], type table}}),
    CombinedT = Table.Combine(GroupedRows[T])
in
    CombinedT

Here's one without an index if the date column contains proper dates:

 Table.AddColumn(ChangedType, "New Best Score", each if [Score]>List.Max(Table.SelectRows(ChangedType, (x)=>x[Date] < [Date] and x[Name]=[Name])[Score] &{0}) then "X" else "")

where ChangedType is the previous step.

For pasting into Advanced Editor, includes extended source data with an extra name, conversion of textual dates by adding year data and converting to proper dates:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WclTSUXJJTVYwNAIyDi2wNFCK1UESNQaLWpiiippCRC1QRI3MICaYoIqaQ9Qag0WTsNqWhGybpRGqGIpdSVjtSsK0KxYA", BinaryEncoding.Base64), Compression.Deflate)),{"Name","Date","Score"}),
    AddedPrefix = Table.TransformColumns(Source, {{"Date", each "2024 " & _, type text}}),
    ChangedType = Table.TransformColumnTypes(AddedPrefix,{{"Date", type date}, {"Score", Int64.Type}}),
    AddedCustom = Table.AddColumn(ChangedType, "New Best Score", each if [Score]>List.Max(Table.SelectRows(ChangedType, (x)=>x[Date] < [Date] and x[Name]=[Name])[Score] &{0}) then "X" else "")
in
    AddedCustom

The order of the source data doesn't matter.

It would probably better if there was a Table.Buffer in the query too.