My bet is Signal for really secure chats.
For general public Telegram chats are secure enough (but I've heard some specualtions that Durov gave access for FSB, russian security service).
I just heard of Session, which is what let me to bring this up.
I've always used Telegram previously since it seems easier for non-technical people to on-board. However, after hearing this, I think I'm going to start suggesting Session instead. Thanks for the info!