Slackware. Once you figure out how to configure everything by hand, any other distribution is cake.
I run Fedora. It is similar to CentOS as it is the upstream distro for CentOS/Red Hat. Mainly, I run it due to familiarity, but also because I am a RH admin for work, so it keeps environments pretty close.
Twenty years ago I bought a boxed Slackware distro because it had an instruction book with it. Nobody told me it was supposed to be hard to learn. ;)