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.
Signal is much more polished and less fragile than Matrix, but is pretty barebones especially in terms of features for large communities. Matrix additionally has a browser client and many non-Electron desktop clients. It’s really close and depends on the use case imo. For personal messages I think I prefer Signal, for communities Matrix.
I haven’t used Threema.