The optimal wingtip shape for an aircraft depends on several factors, such as its size, weight, speed, range, and mission. However, when selecting the best wingtip device, you should consider the trade-off between drag reduction and weight increase. Wingtip devices can lower the drag of the aircraft but they also add weight and structural complexity. Additionally, you should consider the operating altitude and speed, as well as the wing geometry and design. Wingtip devices are more effective at higher altitudes and speeds, where the induced drag is higher and the vortex is stronger. Furthermore, they can affect the aerodynamics of the whole wing, not just the tip. Therefore, you need to consider the shape, size, and sweep of the wing and how it interacts with the wingtip device. For example, a wing with a high sweep angle may benefit more from a raked wingtip than a winglet since it can increase the effective sweep and reduce wave drag.