I've been working in the instant messaging industry for quite some time already and my recent discovery might be the best one in its niche.
Telegram messenger creators did everything close-to-perfect right. They really put the effort and thoughts into the protocol and the features it provides. Most importantly, everything, expect the server is open and writing bots and clients is actually encouraged.
Now my roster is full of my own bots that allow me to add trello cards, check app store ratings, put todo items and share files between my phone and laptop.
Do try Telegram if you search a for great substitute to WhatsApp family apps.