Custom Lookup

smpa01, 
You can create the custom column with this formula below

"CAT" & Text.From(List.PositionOf({[CAT1Value], [CAT2Value], [CAT3Value]}, List.Max({[CAT1Value],[CAT2Value],[CAT3Value]})) + 1)

Hi smpa01

Try this:

let
    Source = Excel.CurrentWorkbook(){[Name="Table2"]}[Content],
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Index", Int64.Type}, {"CAT1", type text}, {"CAT1Value", Int64.Type}, {"CAT2", type text}, {"CAT2Value", Int64.Type}, {"CAT3", type text}, {"CAT3Value", Int64.Type}}),
    #"Added Custom" = Table.AddColumn(#"Changed Type", "Max_CAT1_CAT2_CAT3", each List.Max({[CAT1Value],[CAT2Value],[CAT3Value]})),
    #"Added Custom1" = Table.AddColumn(#"Added Custom", "Custom", each if [CAT1Value] = [Max_CAT1_CAT2_CAT3] then "CAT1" else if [CAT2Value] = [Max_CAT1_CAT2_CAT3] then "CAT2" else
if [CAT3Value] = [Max_CAT1_CAT2_CAT3] then "CAT3" else null)

in
    #"Added Custom1"

Anonymousthis is awesome mate. Thanks !!!

smpa01,

Actually this is a better solution for the custom column to adapt to your table

{[CAT1], [CAT2], [CAT3]}{List.PositionOf({[CAT1Value], [CAT2Value], [CAT3Value]}, List.Max({[CAT1Value],[CAT2Value],[CAT3Value]}))}