Best Quarter/Year calculation

Anonymous , indeed, it can be achieved by Power Query and DAX.

Power Query solution,

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("Vc5BDoAgDETRu3SNSSlV6DFcEy5hvH8U4iBddPMy+WmtdN5X3IRjoUDxvcLUws8GtpWFP84JLIhIX++ODVxWHpHOebYTIqmvD8cG1pVHZPD8RBFRt1ZE1D2oiGDdHg==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [QtrYear = _t, QtrNumber = _t, SalesQty = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"QtrYear", type text}, {"QtrNumber", Int64.Type}, {"SalesQty", Int64.Type}}),

    #"Grouped Rows" = Table.Group(#"Changed Type", {"QtrNumber"}, {{"Best", each [best=List.Max([SalesQty]), result=Table.SelectRows(_, each [SalesQty]=best)][result]}}),
    #"Expanded Best" = Table.ExpandTableColumn(#"Grouped Rows", "Best", {"QtrYear", "SalesQty"}, {"QtrYear", "SalesQty"}),
    #"Removed Columns" = Table.RemoveColumns(#"Expanded Best",{"QtrNumber"})
in
    #"Removed Columns"

DAX calculated table solution

Best Qtr = 
VAR __t = SUMMARIZECOLUMNS ( Sales[QtrNumber], "Best", MAX ( Sales[SalesQty] ) )
VAR __best =
    ADDCOLUMNS (
        __t,
        "QtrYear", CALCULATE ( MAX ( Sales[QtrYear] ), Sales[SalesQty] = EARLIER ( [Best] ) )
    )
RETURN
    __best

DAX measure solution

Best Sales = 
VAR __best =
    MAXX (
        DISTINCT ( 'Sales'[QtrNumber] ),
        CALCULATE ( MAX ( Sales[SalesQty] ), ALLEXCEPT ( Sales, Sales[QtrNumber] ) )
    )
RETURN
    IF ( MAX ( Sales[SalesQty] ) = __best, __best )