Signal by Open Whisper Systems is the best.
It doesn’t have the Hangouts embedded goodness such as video calls and audio calls but it is more than competent at being a text based chat that is properly encrypted.
Can act as SMS as well.
Properly encrypted packets not like iMessage which encrypts then signs each packet (making the signing of each packet useless).
Saves chat history and you can encrypt and export a chat log which can only be decrypted using your private key / cert which is only known to you (like it should be).
The keys are not known or accessible by the company unlike Apple everything or with Google’s Allo when using the non private setting. Even though WhatsApp uses Open Whisper system’s Signal protocol, unless you ticked a box during the June/July all messages can be read by Facebook who must have your key to do so. Prior to that June / July period Facebook didn’t meddle with WhatsApp security. Now it’s useless as even though you may have denied Facebook access to your key other people would not have known about setting this option. Many muggles use it thinking that Facebook is not involved when in fact they are “improving your Facebook advertisement experience”.
Telegram has a very nice front end and hyped up their security but they rolled their own encryption and their protocol was found to be severely lacking.
Haven’t used the other ones yet. Wickr is supposed to be good but the chat is ephemeral.