Always a classic, made popular by a certain fictional spy who liked his shaken and not stirred, the Martini has gone down in history. There are plenty of variations, and even debate as to how a classic Martini should be made (the ratio of gin to vermouth varies wildly).
70ml London dry gin.
15ml Dry vermouth.
Pour all the wet ingredients into an ice-filled shaker and shake well (or stir, if you must). Strain over a martini glass and garnish with a green olive.