Probably should have done this with a CTE, but anyway, I think this works:

SELECT mx.SaleYear, mx.SaleWeek, mx.highestSale, sd.ProductID

FROM

(SELECT SaleYear

, SaleWeek

, MAX(Quantity) AS highestSale

FROM SalesData

GROUP BY SaleYear, SaleWeek) mx

INNER JOIN SalesData sd ON (

mx.SaleYear = sd.SaleYear

AND mx.SaleWeek = sd.SaleWeek

AND mx.highestSale = sd.Quantity

)

ORDER BY sd.ProductID

, mx.SaleYear

, mx.SaleWeek;