The shape of diamond you choose is very much a matter of personal taste, or the taste of the person you are buying it for.
The traditional diamond shape is round. Since a round diamond is symmetrical and capable of reflecting nearly all the light that enters, it has the greatest brilliance of all shapes. The six most popular "fancy shapes" are marquise, emerald, princess, pear, oval and heart.
A round diamond may appeal to a more traditional woman, but a heart shape has greater appeal to the hopelessly romantic.
Another consideration, when buying a diamond ring, is how the shape of the diamond will look on the hand of the person you are buying for. A woman with a small hand may prefer a round or princess shape diamond. A woman with long, slender fingers can wear a slightly longer marquise, pear or oval shape diamond without having it cover too much of her finger.