What is the product that has the highest average rating from the mountain product line?
mountain product line refers to ProductLine = 'M'; highest average rating = max(divide(sum(Rating), count(ProductReview)))
SELECT T2.Name FROM ProductReview AS T1 INNER JOIN Product AS T2 ON T1.ProductID = T2.ProductID WHERE T2.ProductLine = 'M' GROUP BY T2.Name ORDER BY CAST(SUM(T1.Rating) AS REAL) / COUNT(T1.ProductID) DESC LIMIT 1