Signal : needs phone, you can use 5sim or rent a number to get access to the account. keep in mind, having a number rented off internet can lead to problems in the long term, always get a backup. Signal is e2e encrypted.
link : https://www.signal.org/
Molly : Forked signal, privacy enhancements & other features.
link : https://molly.im/
Session : Session works like tor , it routes your traffic through 3 different “relays”, and is e2e encrypted. It does not even need a mail or phone, you’ll get a session id (works like mullvad) Session is also minimizing risks of metadata exposure
link : https://getsession.org/
XMPP / jabber : top encryption, decentralized, open to eveyone. also everyone can run their own server. You can use a server, and add your own protections like OTR, OMEMO, TOR to reroute the traffic while avoiding exit nodes.
Notable others : pdigin, profanity etc…
Recommended plugins : OTR (Off-The-Record), OMEMO, OPENPGP (if you need to secure the messages informations, BEFORE, sending it). All available on app plugin tab directly
If you can use these apps, but need even more privacy, make sure both ends use PGP, not even a compromized server logging message could decrypt the message
Recommended pgp tools :