When choosing between telegram, viber and WhatsApp:
All of these have desktop versions, which is very neat. You can type from your desktop/laptop and it gets cloned to the phone/tablet.
However, WhatsApp must have an active connection from the primary device (phone) for the desktop to connect.
Viber does not need an active phone connection, you can turn it off.
Telegram works the same way.
The other disadvantage WhatsApp has, you can only install it on one additional device, not multiple desktops/tablets. If you install it on a second device, it will deregister on the first. So you get one phone and one computer.
Telegram and viber get multiple devices. However, viber has one advantage over telegram, you can make video phone calls from the desktop, unlike telegram that can only make voice calls.
One really neat feature of telegram is that you can give your contact info to someone without giving your phone number. You create a nick and share that. Very convenient.
Signal is great in US but not used very much outside of US.
WhatsApp is the most popular, with viber being a close second and telegram is not as popular as either.
My favorite is either viber or telegram.