More than a replacement

With the current rate of advances in prosthetics and bio-engineering it is probable that prostheses are built that surpass the capabilities of the original body part. We are seeing the first signs of this in the Flex-Foot Cheetah, where the runner needs less energy to run at high speeds, albeit losing mobility.

Artificial organs are being used to bridge the time to transplantation, but permanent implants are possible in the future.

These days prosthetics are not advanced enough to merit replacing a healthy, fully functioning body part, but that may not be the case forever. Which human body part will be the first to be voluntarily replaced by a prosthetic, and why?