So, I have a Threema license, but from what I’ve seen its encryption isn’t post-quantum. Signal’s encryption seems the strongest. I host my own matrix server.
Also, I kind of don’t care where the servers are or which provider it is. Everything is encrypted anyway.
Self-hosted Matrix.
It still needs polish, but it’s on a good path. Meanwhile others are centralized by a single authority with an easy target painted on them for government coercion along with multiple other attack surfaces, and even information easily traced to PII. Also, not everything is encrypted. A lot of metadata is left out of E2EE. And those servers/providers have that data.
By contrast, a drop in the ocean is far more likely to not be targetted from the outset, making pretty much any matrix server (potentially with the exception of the matrix.org one, but it’s ok to treat it as a demo anyway) a really good choice in that sense.