The main site https://xmpp.org/software/ lists lots of different options but I have no idea what core/advanced means and comparing all of these would take ages.
That software list, how it's done and how it's ranked is literally confirming my initial point of critique :D
Last time I tried out several chat clients, most of them were alpha software, had lots of bugs appearing in normal conversation flows, well, or were so broken that they broke compatibility in subminor version updates to their very same client apps.
I just wish there was some kind of ACID test suite for XMPP or something else to reproducibly validate spec compliance. Maybe a test server or similar as a reference implementation. This way client or server maintainers would have to run their programs against the official test server to increase their compliance stats.
This is exactly what the Compliance Suits are for, and the XMPP Software Fundation is taking care of telling all the clients what they misses directly on the official website, for example: https://xmpp.org/software/movim/
This should not be allowed.
You can't run scripts on all the XEPs declared, some of them are purely redaction or bound to specific UI/UX behaviors. This is based on trust that the developers actually implemented things as stated.
https://joinjabber.org/ is/was an attempt at something more user-focused. It is not linked to the XMPP Software Foundation. BTW, joining the XSF and participating in discussion around protocol evolution, communication strategy and these sort of things is free, and only requires asking for write permission on the XSF wiki to add an application page. Everything happens in the open (mailing lists, chat rooms). We value democratic processes.
Not sure about servers, but for clients there's Gajim, Dino, and Conversations. Not much else is super relevant these days. Profanity exists but is significantly worse than irssi or weechat despite looking superficially similar. Kaidan is a KDE/Qt alternative to Gajim but I'm not sure if it's usable yet. It may be worth switching when it's fleshed out to escape the bugs and slowness of the GTK-based clients.
[0]: https://snikket.org
The developer is very active in updating and maintaining the software (both client and server), and it already supports most of the XEPs.
It's open source and fully supports self-hosted as first class, but if you want to support the developer he offers a cloud hosting paid offering as well. There's a crossover offer with JMP.chat too. If you pay $5 upfront for your first month of JMP.chat, you can get a free cloud hosted Snikket server for it to be setup on. As long as you maintain at least one number with JMP.chat, you keep the server maintained. If you don't, you get a chance to migrate your data. The Snikket cloud server gives you an XMPP server admin account, and you can setup as many accounts as you want. The caveat is that Snikket implementation is optimized for <1000s of user accounts per server.