The Collins is beautiful for so many reasons. First, unlike other cocktails, you need very few ingredients, all of which are easily attainable. If you run out of angostura bitters, for example, you’re not making old fashioneds without a trek (unless your corner bodega is a LOT hipper than mine) -- but pretty much everybody can get their hands on lemons, sugar, and soda water no matter where they are. Second, and very much in keeping with the Mr. Potato Head school of bartending, you can swap out the spirit for pretty much anything and end up with something delicious.
Old Tom gin is the classic recipe, but London dry gin is delicious as well -- and so is rum, vodka, bourbon, rye, and tequila. That’s because a Collins is basically a sour served tall and topped up with soda; the changes in the base spirit merely showcase the particular attributes of that spirit. Tiny variations in the recipe yield a staggering amount of new cocktails, ensuring you’ll never become bored. A Collins made with lime is a rickey, or, made with lime and ginger ale instead of soda, it’s a buck. Throw mint in the shaker, or cucumber (or both), or float Campari or any kind of bitters on top. Play around -- the base recipe is so solid that most simple variations or flourishes are winners.
Shake all ingredients except club soda with ice in a cocktail shaker briefly but vigorously. Strain into a collins glass filled with ice, top with soda. Garnish with a lemon slice and a cherry, or drink immediately if you can't wait.