1. AMERICAN WHISKEY

What to buy:

Nothing against the exceptional whisk(e)y produced by Scotland, Japan, and Ireland, but when it comes to picking one all-purpose whiskey to keep in stock at home, you've gotta go American. If you can buy two bottles, go with a good bourbon and a rye as well. For the former, standard Buffalo Trace works if you want something a little more subtle. Four Roses also makes a great product for the money. These days though I'm a big fan of mixing with Bonded Old Grand Dad. For around $20, it makes a killer Old Fashioned.  As for rye, buy Rittenhouse if you can find it or Wild Turkey 101. George Dickel is pretty damn good for being priced just above free. 

How to mix it: 

Old Fashioned. Whiskey Sour. Sazerac. Manhattan.

2. LONDON DRY GIN

99% of the time I drink gin, I'm drinking it in a cocktail. It's a spirit that I love, but almost never on its own. And that's an important thing to learn right there. Just because you don't love the unadulterated flavor of a liquor, doesn't mean you won't love what can be made with it. I don't care what you think about gin. It's absolutely essential for your home bar. 

What to buy: 

Go classic with Beefeater or Tanqueray. Martin Millers if you are intent on making a lot of martinis. Eventually buy some Old Tom gin.

How to mix it:

Kick James Bond in the nuts and make a proper martini. Never be not drinking a Negroni. Whip up a Gin Fizz if only to learn why that one bartender stared at you with the eyes of death itself the last time you ordered one during a busy brunch. Tom Collins. Last Word. Bees Knees. Corpse Reviver #2.

3 & 4. Sweet and Dry Vermouth

Most people think vermouth is gross because most people have only ever tried gross vermouth. The problem of gross vermouth is two-fold: 1) some vermouth starts out bad 2) all vermouth will eventually go bad.

The first problem is easily solved. Buy Dolin for the dry stuff. Buy Cocchi for the red. Dolin red is ok, but I don't love it. I really want to like Martini Rossi because they sponsor the Williams F1 team and fast cars are just the best, but I think it tastes like pizza in a shocking first and hopefully last ever negative comparison to the flavor of pizza. At home I use Punt e Mes because that stuff is just absolutely delicious, but I do acknowledge that it's a bit aggressively bitter and does subtly change the flavor of a lot of classics. I just think it changes the flavor for the better is all. Carpano Antica is for fancy people who like vanilla. I occasionally enjoy being a fancy person who likes vanilla. Just not always.

Regarding that second point, it's important to remember (or learn) that vermouth is made from wine, and like wine, is susceptible to oxidization. The best way to prevent your vermouth from oxidizing is to get to the bottom of the bottle faster than oxygen can get to wrecking it (two months or so). If it'll take you longer than two months to finish a bottle, first consider buying a smaller bottle. A lot of vermouth is coming in 375ml bottles these days. This is not a time to indulge your Costco impulse. Buy the smallest bottle you can. Next best option is to portion out a new bottle into smaller bottles, filling them all the way to the top and keeping them in the fridge. I use a vacu-vin at home and like how that works as well. 

What to buy: 

See my above answer to problem number one.

How to mix it:

Start by drinking it all on it's own, on the rocks, maybe with a twist and some soda water. There, you like vermouth now. Also, seeing as how it's well under 20% abv, vermouth on the rocks is one of the best ways to hang out and drink with your friends without getting hammered. Keep that in mind. As for the dry stuff, make a martini. Actually, make many martinis. Use different ratios until you dial in what you like (4 to 1, gin to vermouth is pretty damn great). When it comes to sweet vermouth, make a Manhattan, realize how easy a good Manhattan is to make, then revel in that fact while reclining on your couch like some sort of non-sexist Don Draper. Make a Perfect Manhattan to learn that both vermouths can work together quite nicely. Old Pal in the summer. Boulevarier in the winter.

5. Bitters

I keep wanting to say that bitters are like the salt of cocktails except that they totally aren't at all. For one thing, salt is the salt of cocktails. Secondly, bitters and cocktails don't have the "absolutely necessary in everything" relationship that salt has with food. Lots of great drinks don't have bitters and are the better for it. What bitters do well is to help balance and unify the flavors of the entire cocktail in which they are used. Just a tiny dose, a few drops, can make a huge difference, adding character and depth to an otherwise unremarkable drink. With that in mind, I guess a better analogy would be to say that bitters are like the salt of cocktails. 

What to buy:

Angostura. Peychauds. Orange (from my vantage point, Regan's #6 is sort of the industry standard). 

How to mix it:

A few dashes should suffice. Ango in Manhattans. Peychauds in Sazeracs. Orange in Perfect Manhattans. All three or a combination of in Old Fashioneds. 

Ok done. It's worth stopping to note that you currently have two bottles of spirits, some vermouth, and bitters in your bar right now. That's in. And guess how many delicious drinks you can make with that paltry offering? Hundreds. Seriously. Start mixing in various syrups and juices and you will never not have something delicious to drink. In only five smartly chosen bottles, you have a very usable bar. Well played.