Signal is solid, though a few privacy advocates criticize the dependence on Google Play Services on Android phones. I also get the impression that development has slowed, with the developers (Open Whisper Systems) focusing on implementing the Signal Protocol in other clients (but correct me if I'm wrong).
There's also the newly open-sourced Wire - it has the same dependence on Google, but it supports video calling and some might prefer their UI.
There's also the newly open-sourced Wire - it has the same dependence on Google, but it supports video calling and some might prefer their UI.