NixOS. Only been running it on that server for a couple of weeks now, but so far I'm happy with it.

I like being able to manage almost every aspect of the system from a single declarative configuration file.

I don't do any fancy NAS and/or RAID stuff though. Just the OS living on an nvme, and a logical volume running across two spinning disks. If I need direct file access I use scp, but the storage is more for jellyfin and a syncthing node.