Hello friends,
Threema is good for this (swiss servers, encryption, etc. ). Downside it is paid, which makes it harder to get everyone on the app.
Session is really good (no phone number, encrypted, minimizes metadata).
Briar is unbelievable. It sends messages over TOR and is decentralized. No single point of attack, really anonymous. It also has capabilities to send messages offline (over Bluetooth or other options for dead-drop style communications). Downside is android only.
XMPP is really good. You can use encryption like OTR or OMEMO (better), you can also run your own servers. There are many client apps for every device. Downside is harder to use, better for a technical crowd.
Matrix is similar to XMPP but not as good (more meta data). But easier to use.
If you require top security above those levels there are other much better systems to use that are much harder to set up.