List of fonts that you will be useful when designing apps:

Apple San Franciso

San Francisco Compact is the font for Apple Watch.
San Francisco is the font for iOS after iOS 9.

https://developer.apple.com/fonts/

Android Roboto and Noto

Roboto is the latest system font after Android L (5.0).
Noto is the corresponding international font including traditional and simplified Chinese.

https://www.google.com/design/spec/resources/roboto-noto-fonts.html