Speyside has the highest concentration of distilleries in the five regions. over forty distilleries are clustered within the Spey valley. (the river Spey is Scotland's second-longest river, 110 miles, and its fastest-flowing.) four of the top five best selling single malts are from Speyside: Glenfiddich, Glenlivet, Macallan and glen grant. Speyside has no one particular type or style of whisky, with bourbon and sherry casks both commonly used for maturation. what the vast majority of Speyside whiskies have in common, however, is the absence of strong peat flavour.