Doesn't matter. Any distro that's worth using is just fine as a server. Just use whatever you use on a desktop to minimize niggly almost-the-same-but-not-quite annoyances between them, and you'll be happy.

Personally, I like Debian Testing on my servers -- it's reasonably recent (as opposed to Debian Stable, which gets hopelessly outdated quickly), has no releases (instead it just stays up to date as you update stuff) and is rock solid for me, but it's more a matter of taste than anything else.

Besides, I like Debian's attempts to mostly stick with Free software in their base distro. While I'm not a fanatic, I like knowing that what I use is open source, and that I can fix it and debug by hacking it up (and occasionally have) if it breaks on me.

BSD?

runs

LFS.