Some shapes are better suited to certain setting styles. If the setting is very important to you, that can drive the shape of the diamond you select. If you prefer a solitaire setting, you can choose any shape available and show the diamond off to its best potential.
Oval, round, princess, radiant, cushion or marquise shapes are well-suited to being the primary stone in a multi-stone ring. Asymmetrical shapes such as the pear, trilliant or heart are best displayed in solitaire settings. They are difficult to balance with other stones in a multi-stone ring, but trilliant shapes work well as accessory stones.
So what diamond shapes work best for you? Consider the hand that will wear it, the other parts of the ring, your taste and your lifestyle. These are the most important factors when choosing a ring that you will cherish for a lifetime.