The Margarita is regularly the most widely ordered cocktail in the Nation. While this of course has a lot to do with how delicious it is (when properly made), I suspect it is also one of the few cocktails that is tied tightly to a particular cusine (Mexican). Any time you order a drink in a Mexican restaurant, the Margarita always seems like the right choice.
The basic ratio for a Margarita that I've found works well, is 3 parts tequila, 2 parts Cointreau, and 1 part lime juice. It of course requires you to use a good tequila.
Garnish: lime wedge, and optionally pre-salt the rim of the glass.
Shake with ice. Strain into cocktail glass, or an ice filled rocks glass.