For privacy session can't be beat. So, whats wrong? Simplex: Leaks your IP and needs the host to be online. Signal: Not decentral Bitchat: Thats more like IRC than DM and its location / relay based. Nostr DM's: Various standards and if done wrong your metadata is very public. That leaves session. Session is completely metadata secure but doesnt persistently store messages server side. Thats a plus and a con. It is decentralized although with a centralized bootstrap. Another one I personally use is Matrix. Element is a nice client for it but if you use Matrix avoid the main orgs server as its subject to UK law. You want a third party one, they all federate. On Matrix whatever homeservers are involved have the metadata but its end to end encrypted.