Is there a recommended or "blessed" server and client combo for someone who just wants to migrate their friends off discord?

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.

>Is there a recommended or "blessed" server and client

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.

The targeted audience of this website is, for now, developers. Communicating is hard.

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.

If you stick with mobile use, there is Snikket[0], which provides a branded server+mobile app ecosystem that should "just work". YMMV; I haven't tried it myself.

[0]: https://snikket.org

Core and advanced meant the compliance suites.[1]

[1] https://xmpp.org/extensions/xep-0479.html

Monocles chat is best one for Android, it is the most complete chatting app for todays standards, unlike Conversations it has swipe to reply, last seen, emoji reactions etc. The only issue is making account there, need to use other homeserver like @conversations.im if you don't want to pay for their @monocles.eu . For IOS the only option is Monal. For web I find conversejs better than mov.im as movim doesn't encrypt sent pictures in chat at all, and encryption of text messages is sometimes broken depending on how you set it up in settings of account and in chat, as it needs to be activated on both places, so conversejs is better, but less enjoyable UI than movim

The ironic part is that those software description files are meaningless. AstraChat claims Advanced in all categories, but it's a proprietary commercial software, so nobody ran any kind of test suite to verify this.

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.

Conversations is a great Android client (also brings their own backend instance if you don't want to host your own), I don't know about iOS or server though.