The classic diamond shape, a round stone with 58 facets, has the most sparkle.
The heart, oval, and pear resemble their names (a pear diamond worn with the point facing away from the body makes the finger look long and slender). The marquise is an oval with pointed ends. The princess is a brilliant square stone. An emerald cut (originally developed for the gem of the same name) is rectangular with cropped corners and long, stair-step-like facets; Asscher is similar, but square, so it looks octagonal (the radiant is a popular variation of these but with facets that give it even greater sparkle). The cushion cut is a cross between a rectangle and an oval; it has a pillow shape.
Heart, marquise, and oval rings by Tiffany & Co.; princess, emerald, and cushion rings by DeBeers; Asscher ring by Daniel K; pear ring by Asprey