[herbig]

Most people here know about Signal, but if you don't:

https://play.google.com/store/apps/details?id=org.thoughtcri...

Open source, easier to use, much better UI.

[cyphar]

And it uses Axolotl (which is an improved version of OTR) which has much better security properties for informal messages (the recipient of a message can be convinced the sender sent it but could not prove this to anyone else). PGP should only be used in cases where you are willing to have your signature be made public.

[neoCrimeLabs]

Signal also has forward security, last I checked.