This will be a purely personal answer, as we don't really maintain any official list of favorites.

Myself and my family are running Fedora's KDE edition. The Fedora team has a long history of working very closely with the Plasma dev team, quite actively contributes upstream, and I haven't been disappointed. I'd vouch for this one from first-hand experience!

We also have a new project to produce a distro of our own in the works, called KDE Linux. That has recently had its first alpha release. It still has some real feature gaps and may not serve you well if one of the missing bits is something you require, but it's definitely worth looking into. It has a lot of next-gen ideas baked and some things we got to learn during the SteamOS effort, and think it has a place in the ecosystem.

In the dev community I generally see a lot of people running KDE on Arch, Debian and openSUSE as well.

My usage of KDE today is on the Steam Deck. I actually did take the time to set it up to look nice.

I have three observations to make.

1) There is a pain point in KDE involving windows on top of another fullscreen window. Specifically, the Live Captions app. This is a design problem that other OSes like Microsoft Windows doesn't suffer from. What happens is, if you leave the taskbar in place and want to have the captioner running over video and make the video fullscreen, you can't with KDE. I eventually figured out a hacky workaround in that if I turn auto-hide on for the taskbar, I can then alt-tab the Window and it'll stay on top. I did see some talk about this window behaviour. It's nuts that an accessibility feature needs the user to make the taskbar auto-hide first before it works. It would be nice if there could be a setting where it is "this window is on top of everything".

2) SteamOS as having a large device-specific installed base, I think there is value in the KDE team encouraging Valve to turn KDE from a snapshot to a rolling release as Fedora has done where KDE is rolling but the rest of the distro is a snapshot. Why this matters, is because when it comes to bug reports and the like, the KDE bot basically closes them because Valve's KDE is "unsupported" because they only seem to rebase once a year or two. I know they did move to 6.2 with 3.7, but I found 3.7 buggy and thought KDE looked worse and had worse scrolling performance (maybe setting conflicts from the 5.27 version?) so I just switched back to the 3.6 version with 5.27 where everything looks right and scrolling moved smoothly. I know they changed the kwin compositor a lot in the 6.x series, and I suspect there are regressions in it, so if it isn't rolling, people are stuck with a buggy compositor? That sucks.

3) Setting upgrades.. so with SteamOS, Valve was shipping the 5.2x series of KDE. Now 6.2, and the next rebase is probably going to be 6.x on Wayland instead of X11. So there's a lot of legacy cruft settings that carry on and I think they cause glitches. I always used to do clean install of Windows versions, and think it would be nice to be able to do that in KDE, but there's a lot of legacy cruft with various settings and conf files scattered in various random folders. At some point it really should be cleaned up.. one central folder for all settings.. no some settings are stored in a KDE4 folder, and others are stored in this other one KDE5 introduced, etc.

Thank you. After hearing about KDE Linux here on HN I'm now very interested in the project and its future.

Personally I've had an issue with KDE on Fedora several years ago, possibly due unstable Wayland, but I don't know real reason. Something in the graphic stack failed. So that was a reason for me asking about it.

What's the relation of KDE Linux with KDE Neon? The former was made by Blue System and the new one by Techpaladin or something like that?