‘Seems to always be a Debian variant of some kind, depending on the specific application… While I have administered Red Hat/CentOS servers, I enjoy, and am most familiar with distributions that descend from the Debian side of things. I’m sure it’s possible I’d have a different perspective if I had came up in the Red Hat world, but I really think apt, and systemd/init are great.
For anything with extra security requirements, Alpine Linux is often the first place I go.
For storage, if I’m setting up something special or custom (like a GlusterFS cluster), I’ll likely first look to Debian itself (without a desktop environment), or an LTS version of Ubuntu Server. But (even though I know it’s Unix and not Linux), for anything from a SMB NAS, a File Server utilizing Directory Services, or a large scale NFS Server, my first choice would be TrueNAS (formerly FreeNAS).
All of these (with the exception of the few Unix variants) are Debian-based Linux distributions.