Since nobody has said it yet: Ubuntu server or Debian. /s
In seriousness, I use both. They’re pretty great. Note that if you use Ubuntu server you can get Ubuntu Pro free for up to three devices. This comes with ten years (!) of security patches. Great if you aren’t keen on upgrading anytime soon.
Debian or Ubuntu Server (or something specific to servers purpose, like OMV, etc).
… but ProxMox (a hypervisor, Debian based) doesn’t have much overhead & runs on old PCs pretty well. And with that, you can pretty much try any distro (as a full virtual machines, perhaps with dockers within it, or as a lightweight containers that are really resource efficient). Or separate containers for each purpose (for beginners, there are like TurnKey solutions to stuff like NAS, it takes literally a few minutes to set up).
Backups (snapshots) are easy too, and a later migration to a better/next server is basically two clicks away.
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.
Linux is quite lightweight. Pick a distro that doesn’t run a lot of stuff by default. OpenBSD only runs sshd exposed to the network, AFAIR. Debian probably does the same. But really, the lightness comes from what isn’t running. NixOS, fedora, rocky, alpine are all decent alternatives.
I use arch.
edit: lol while I am new to arch, I guess I kind of expected people to disagree with me. I was under the impression that stock arch is very lightweight? I know there used to be jokes about “I installed Arch” cause it’s supposed to be hard. But I installed Arch on my desktop and server recently, I did the manual install on my desktop and the guided install on my server. Both super straight forward. Plus Arch seems to have some of the best documentation across distros. I don’t know why it should not be suggested, unless I am missing something.
Most people want stability (low change) for servers. Arch is typically run where plentiful software updates are welcome. It’s not that you can’t/shouldn’t use Arch for servers, but it isn’t the most conventional suggestion.
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