From the website:
Use anything by Open Whisper Systems.
Edward Snowden, Whistleblower and privacy advocate
Signal is the most scalable encryption tool we have. It is free and peer reviewed. I encourage people to use it everyday.
Laura Poitras, Oscar winning filmmaker and journalist
“I am regularly impressed with the thought and care put into both the security and the usability of this app. It’s my first choice for an encrypted conversation.”
Bruce Schneier, internationally renowned security technologist
After reading the code, I literally discovered a line of drool running down my face. It’s really nice.
Here’s a comparison between Signal, Whatsapp and Allo. https://theintercept.com/2016/06/22/battle-of-the-secure-messaging-apps-how-signal-beats-whatsapp/
And some security tips for Signal https://theintercept.com/2016/07/02/security-tips-every-signal-user-should-know/