Eggplant come in a dizzying array of shapes and sizes. There is an oval off-white variety that it probably seemed sensible to name eggplant, there are tiny green eggplant prized for their bitterness, and there are the more widely available varieties we concern ourselves with here: the long, slender, usually lavender “Japanese” or “Asian” type and the more common dark purple Mediterranean breed.
Though you might follow geographic clues for which to use—pick the Asian type for Middle Eastern and Asian recipes and the globe eggplant for Western European recipes—I think the lavender variety is generally so superior (unless you have a garden or gardenlike source, in which case they’re all great) that I select it whenever I have a choice. In general, the smaller the eggplant, the better (I love the golf-ballsized ones, which do not require any trimming or cleaning), but—also in general—firmness is the most important characteristic when buying an eggplant (after, of course, obvious lack of damage).