It would be irresponsible to start any list of the Most Important Cocktails without first mentioning the OG classic, the Old Fashioned. As early as 1806 there are references to the “Bittered Sling”, a morning tippler comprised of spirit, sugar, bitters, and water a combination that would span centuries and be among the first to bear the name “cocktail”.
The first documented appearance of a recipe that we would recognize was introduced as the Whiskey Cocktail in How to Mix Drinks or the Bon Vivant’s Companion by Professor Jerry Thomas. This original incarnation would curl the modern mixologist’s moustache in all the wrong ways, as it was shaken and served up in a wine glass. But this was only the beginning. This combination would evolve into the Improved Cocktail, which used posh European ingredients, such as chartreuse, maraschino, and absinthe to get fancy. This might have been fine for youngsters like your great grandfather, but real gentlemen, like your great-great grandfather were more comfortable ordering their whiskey cocktails in the old-fashioned way, which gradually morphed into the more recognised recipe at the end of the 19th century.
As Robert Simonson mentions in his book, The Old Fashioned, this recipe is the “embodiment of the cocktail” and in the “many variation that have followed, there is liberty and invention.” In the world of cocktails, what other recipe can hold a claim to that?