What is the best free math app?

What is the best free math app?
10 Free Math Apps for Students that Teachers Swear By

Prodigy Game. Meant for students of grades 1-8, Prodigy Game is available for free on iOS, Android and web platforms.
Math Training for Kids.
CK12.
Colorado’s PhET.
Photomath.
Khan Academy.
GeometryPad.
BuzzMath.

10 Free Math Apps for Students that Teachers Swear By
  1. Prodigy Game. Meant for students of grades 1-8, Prodigy Game is available for free on iOS, Android and web platforms.
  2. Math Training for Kids.
  3. CK12.
  4. Colorado’s PhET.
  5. Photomath.
  6. Khan Academy.
  7. GeometryPad.
  8. BuzzMath.