You're working behind a bar and a self-professed "whiskey connoisseur" asks you for an Old Fashioned. Which whiskey do you use, and why?

The Old Fashioned whiskey is actually a cocktail.  In 1881, the recipe was said to have been invented by a bartender at the Pendennis Club, in honour of Colonel James E. Pepper, a prominent bourbon distiller, though there is belief that the recipe was being used before this date.

To answer your question, if they really were a "whiskey connoisseur" then you should use a bourbon whiskey as this was in the traditional recipe, but as with everything in life, the recipe has evolved and can now be used with many different spirits.

The recipe

Taken from George Kappeler and his book published in 1895.  The recipe is:-

1. Dissolve a small lump of sugar with a little water in a whiskey glass
   
2. Add two dashes of Angostura bitters
3. Fill whiskey glass with ice and a piece of lemon peel
4. Add one jigger of whiskey (44ml)
5. Mix with spoon and serve, leaving spoon in glass.

Modifications

The bourbon whiskey can be substituted by Irish or Rye Whisky as these were both readily available in the 19th century America.  In some regions, Wisconsin Brandy was also used, and was subsequently called a Brandy Old Fashioned.

Common garnishes are now orange peel or maraschino cherry. The sugar is normally pre-dissolved into a simple syrup, which is faster to use than granulated sugar and eliminates the risk of undissolved sugar in the drink, which can spoil the drinker's final sip.