The main idea behind a well-stocked bar is two-fold: 1) the majority (say 90%) of cocktails and drinks are variations of a few recurring combinations and 2) having a half-dozen-to-a-dozen spirits on hand, plus some common mixers allow you to make to order a good number of cocktails or mixed drinks your guests might want.
We can imagine a range of bars, from the full bar that a drinking establishment will have to a well-stocked bar for the home to abbreviated, even minimal liquor cabinets. Most likely a complete bar will be out of the reach and even interests of most home bartenders. Traditionally, then, the well-stocked home bar consists of the following:
Classical Well-stocked Bar
Spirits: Vodka, Gin, White Rum, Tequila, 3 Whiskeys (Bourbon, American/Canadian, blended Scotch).
Mixing ingredients: Triple Sec, dry Vermouth, Rose’s lime juice, sweet Vermouth, Angostura bitters, Grenadine
Garnishes: Cocktail olives, Maraschino cherries, Bar salt (kosher salt works), Cocktail onions
Mixers: Tonic water, Club soda, Soft drinks (Coke, Sprite or 7-Up, Ginger Ale), Orange, cranberry, tomato, and grapefruit juice
Fresh ingredients: Lemon and lime for garnish, mint leaves
Each is listed in order of frequency of use.
So is this what you should buy? Depends.
On budget first of all. Even if you don’t go for top shelf, buying fifths of all the above will cost a pretty penny. Say you’re a struggling twenty-something or a sometime cocktail drinker who wants to have spirits on hand when friends drop by but doesn’t feel the need to be a completist. I can imagine you getting away with the following basic home bar, in order of purchase:
Basic Bar
Vodka, Gin, Triple Sec, dry Vermouth, Bourbon, and White Rum or Tequila, sweet Vermouth
Fresh lemon and lime on hand, Tonic Water, Cranberry, Olives and maraschino cherries, Rose’s Lime, Club Soda, Orange Juice
Or, if you’re being particularly budget-conscious, try:
Minimal Bar
Vodka, Gin, dry vermouth, tonic, cranberry, Triple Sec, olives, Rose’s, soda.
It’s barebones, but there are still a number of drinks you can make with that.
Just as important as budget will be the crowd you’re serving for. American, Canadian, or Irish whiskey drinks and scotch drinks are not nearly as popular as they once were, and if your friends are under the age of 40, you may be able to safely omit. You’d be better off buying a single malt scotch for your scotch-loving friends than trying to accommodate the rare Rob Roy, Rusty Nail or Presbyterian requests.
Finally what you stock will vary by what you like to drink and serve. I love sidecars, so I keep brandy on hand. If you like Alexanders or Chocolate Martinis, you’ll stock crème de cacao. Et cetera. As a general rule of thumb, if you’re looking to build on the well-stocked bar, I’d add brandy, Grand Marnier, and two or three cordials of choice, including one sweet liqueur (Kahlua, say) and one bitter (such as Campari).
Ultimately these rules of what to stock are there primarily to guarantee you can make a cocktail or drink to order. The well-stocked bar is a great enabler for entertaining, particularly for occasions when you have more formal affairs or are entertaining guests beyond close friends.
But that’s the only reason for the rules, such as they are, and they can be broken without much harm. After all, you know your guests better than anyone.