H.264 and VP8 are available essentially everywhere. Reasonably recent versions of Android and iOS (5.0 and 11, IIRC) include support for H.265 and VP9. The APIs on both platforms make it very easy to query available media formats, including hardware acceleration capabilities. Hardware acceleration is particularly important on mobile for power consumption reasons, so you should use it wherever possible.

If any app developer believes that they are better able to implement a secure codec than Apple or Google, I have a bridge to sell them.