Martini - 4 ounces gin, 1 ounce dry vermouth; stir
with ice for ~30 seconds, strain into a chilled cocktail glass
Bronx - 2 ounces gin, 1 ounce orange juice, 1/2 ounce sweet vermouth, 1/2 ounce dry vermouth; shake with ice, strain to chilled cocktail glass
Tom Collins - add 2 ounces gin, 1/2 ounce lemon juice, 1 teaspoon sugar to a Collins glass (the tall, skinny kind) with ice; fill with club soda, stir until sugar is dissolved
ETA: Someone mentioned a Ramos gin fizz. IMO, those are only worthwhile if made by someone else, because they require so much work. When made properly, though, it's one of my favorite cocktails.
This post was edited on 2/16 at 3:18 pm