Signal is an okay choice if you don't mind giving away your phone number.
Personally, I prefer XMPP+OMEMO. It works on many platforms and with many clients and you can freely choose your server.
From what I've heard, Wire is also okay, same with Jami (formerly GNU Ring), and there is also Matrix and Briar. Esp. the latter is not sufficiently mature yet, the battery consumption is very high.
If you prefer the e-mail protocol, have a look at DeltaChat.