You're going to get a million answers, mostly people saying to use which distro they're currently using. In my experience, KDE works just fine on any distro that allows you to install it out of the box, so I would choose based on other attributes of the distro, such as:

• Package manager: which are you used to?
• Update cycle: KDE 6 is out soon, so you want something which updates often enough to get it fairly quickly (at least semiannual).
• Stability: unless you want to have to manually maintain your system and learn how it works, avoid arch and arch-based distros. I have run it, its fine, but it's not "normie", and unless you really know what you're doing, daily driving it can be stressful. Manjaro has the same issues, but takes away some ability of the user to fix them.

For instance, I personally like Debian and apt, but I would not recommend base Debian right now, since KDE 6 is about to come out and Debian will take a loooong time to get it. I have not personally used Kubuntu, but if it gets rid of any the bloat canonical has been adding to Ubuntu lately, it sounds pretty good to me.