Selfhosted

It's claimed to be official. But I went with https://continuwuity.org/ since it seemed to have a more active community. Plus ever since then, the core maintainer of Tuwunel has been making threats against Continuwuity including personal attacks, and seems to be quite unpleasant to deal with in general. There's also been a thread about it here. So I honestly lost all taste to reconsider.

That's rough. Signal is the only app that can actually be trusted to resist state monitoring because it has a successful history of it.

I guess another option to throw into the pool is https://docs.cwtch.im/ then. It's new though, and not as easy to use.

As someone who's tried both, it depends on what you want. Your choice of Matrix server depend on any political and ethical values -- some say Synapse is too corporate, being maintained by Element who are for-profit and obtain funding from corps and governments, so some prefer others such as Conduit ( -- until maintaining slowed to near abandonment and it was superseded by Conduwuit -- until the owner got cyberbullied so hard she quit the project and it was superseded by Continuwuity) because it was built on Rust and much more efficient than Synapse, or Dendrite. I recommend Continuwuity.

Then there's clients -- the only mature matrix client for mobiles is Element, and there are two apps, Classic and X, who offer different pros and cons, and imo are not good enough on their own, both are in a kind of beta stasis. But it's the best they have. If you really don't need calling, then Element X, FluffyChat or Schildichat is your app and Element Web for desktop access (available on Github). However, when exchanging encryption keys to trust another of your devices, or a contact's device, only Element offers simple QR scanning.

In short, Matrix is very good as a privacy-focused server with partially working, modern looking clients.

Then there is XMPP. Again there are different backends to choose from and I am inclined to recommend Prosody. XMPP just works out of the box for me, calling included, and is relatively stable. However, there are large caveats -- several pieces of user data are stored unencrypted on the server, which is fine for you as the owner, but it's a lot harder for someone else using your service to trust that. And, while XMPP uses OMEMO encryption keys, handshaking with devices is far more manual than Matrix's Olm/Megolm and involves a multi-step process, and migrating to a new device is a pain because messages are not backwards decrypted, so they must be transferred from the first device. Finally, clients are very rough. The best desktop clients such as Gajim and Pidgin still look like they were built in 2001, and while mobiles have Monocles, Cheogram and Conversations, they all look very similar, as the former are very slight modifications of Conversations.

In short, XMPP may lack some comforts of modern messengers, but it is simpler to set up than Matrix, and offers many of the same features. However, the manual key sharing process might scare off all but the most avid privacy enthusiasts, especially that if you migrate to a new device without sharing message history from a previous verified device, messages are lost.

Choose Matrix for polished software, inviting many contacts, and, with Element X featuring (eventually) Element Call, complete E2EE.

Choose ol' faithful XMPP for an easier initial setup, if video calls are important, you appreciate that historical messages cannot, by design, be hacked into, or if you don't like Element the company.

I too have heard good things about SimpleX and Signal, and recommend trying them if they are valid contenders for your use case. Signal really is the best (most private, least data-farming) non-selfhosted option.

Oh, and if you wish, it's a bit old now but no doubt useful, I have written installation guides on both Prosody and Continuwuity, based on Proxmox containers.

XMPP is ancient. So is email, the internet, and the wheel.