Probably Debian. It’s basically the most used distro, and therefore has many online resources.
Old software, but very stable.
No bloat, very clean.
No custom programs interfering with any configurations etc.
Support for many server software etc.
If you want an even cleaner OS, where (nearly) everything is under your control and as lightweight as possible, Arch would be for you. There’s the bonus of the AUR, but the huge problem of newest, “unstable” software, though I’ve yet to experience any problem on testing repos, except for the Nvidia drivers. In general, Debian should be enough of lightweightiness and control.