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.
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.