Instant messenger and WhatsApp are both owned now by Facebook.
I haven’t used it, but Signal is both encrypted and considered a good app. Telegram is not considered a good app security-wise. Plus, several countries are blocking Telegram for political purposes. The internet works globally, so best to avoid any communication problems.
But for regular messaging, Instant messenger has the problem exactly of being owned by Facebook. My dad refuses to install it because of that. He is just using his phone’s built in messenger. Which does work with Instant Messenger, but without any of the add-ons.