Apple devoted an entire session of last week’s WWDC to Antonio Cavedoni to explain how and why Apple designed their new system font, San Francisco. The presentation is a little dry, especially if you don’t care about typography, but it was very interesting to see someone really take the time to cover all of the little considerations taken into account when making a typeface.
Having used San Francisco across my iPhone, iPad, Mac, and Apple Watch, I can say with some confidence that it is a very good display typeface across a range of devices and sizes.

You have to be a developer to watch the video, but it is worth your time if you are.