My recommendation is Debian for a server (real or virtual), or Proxmox. The former is perfectly reasonable and excellent experience; the latter is more flexible and more complex.

Debian is the parent distro of numerous Linux flavours (including *buntu, which aren't suitable as a server OS, IMHO), so administration and services are all common (apt, etc). No need to learn dnf, pacman/yay, etc.

It's still my preferred server OS, despite other options and being experienced.

Though I do also have a NUC running Proxmox (for VMs and LXCs), and both a NAS and RasPi running Docker. 🤷‍♂️ My Debian server is a VM inside one of them.