Debian. I run Stable on servers and Unstable on desktops.
Although I do think OpenSUSE Tumbleweed and Arch are actually better in some aspects, I find Tumbleweed too rough around the edges (it’s a derivative of Leap and that shows). And I just can’t be bothered to install and configure Arch anymore. Fedora and Ubuntu are too buggy on average, Mint is too “stable” for a desktop and I don’t use all the helpers that make it newbie-friendly. Slackware suffers from issues that were solved in the Linux world decades ago, and I dislike derivative distros on principle.
I’ve probably tried around 30-40 distros and I always return to Debian.