openSUSE. Either Tumbleweed (rolling) or Leap (stable). A few reasons for this:

• Their snapshot system is awesome (they develop snapper) and configured by default when you install it. It takes a snapshot of your system before and after updates and at other times as well. Uses minimal space because they use BTRFS and also have maintenance jobs by default. This is the main factor that made me settle on it. To me, it was the best of both worlds between the stability but restrictive nature of atomic/immutable distros and the freedom but potential for breaking your system of the traditional distros.
• They’re reputed to have one of the best KDE integrations of any distro. I’m not sure what this means in practice because I’m still a newbie, but I’ve seen this mentioned by others a few times. Other major DEs are also options at install time.
• They’re one of the traditional big distros. SUSE has been around since the early days of Linux. SUSE is also a commercial corporation, similar to Red Hat.
• The rolling distro, Tumbleweed has a reputation for being one of the most stable rolling ones because they apparently have an extensive automated testing system for updates.
• Their system configuration tool (YaST) is also supposed to be awesome and one of the best around, minimizing the need to resort to the command line, but I also haven’t used this too much.
• Their community is supposed to be really helpful as well. I haven’t experienced this myself, but have heard it.

Edit: it’s probably best to avoid their “SlowRoll” distro for now. It’s rolling but with less frequent updates than Tumbleweed, which many may thinks is a sweet spot. However, it’s still in beta and I’ve heard of people having issues with it occasionally.