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.