In Pictures: Apple’s most important iPhone upgrade...
I'm including the iPhone camera here as an honorable mention, if only because the camera quality and feature set of the iPhone has steadily improved with each new model. Today, the photo quality made possible on the iPhone 5s is light-years ahead of what the iPhone was capable of even three years ago. Over and above that, Apple has added many great photo quirks over the years, including panoramic photo options, slow-motion video capabilities, and with iOS 8, time-lapse video functionality.
In Pictures: Apple’s most important iPhone upgrades over the years