This is by far one of my favorite gin drinks, unfortunately more then once when ordering this at a bar they had no idea what I was talking about and made me a Sloe Gin Fizz.
2 1/2 ounces gin
1 ounce fresh lemon juice
1 teaspoon superfine sugar or 1 ounce simple syrup
San Pellegrino mineral water
In a shaker half filled with ice cubes, add in the gin, lemon juice, and sugar or simple syrup. Shake well. Strain into a collins glass almost filled with ice cubes. Top off with San Pellegrino mineral water and stir well.
My favorite gin to use in the United States is Tanqueray No. Ten. Another good option for gin is Bombay Sapphire. Bombay Sapphire may not be quite as good as the Tanqueray No. Ten but it is still quite good and here in Switzerland I have taken to using it as Tanqueray No. Ten is much harder to find without a trip to London or at least a higher end liquor store.