Debian should be the standard for servers. Plenty of server packages, support for plenty of platforms. Uses the package manager and package format used by most GNU/Linux users.

Slackware, is the best! very stable, we have no problem for our mailserver for almost 5 years, installed in a P4 clone PC.
Talking about stability, security, speed, and robustness it's proven.

CentOS

I use RHEL, CentOS and Slackware but I think Slackware is the Best

CentOS

I am going with Slackware

For light in-house servers (to run things like git, jenkins, databases etc) I run OpenSUSE. Just as easy to install as my desktop. It has proven to meet our needs and it is very stable.
Running Evergreen versions gets me prolonged updates.

I voted CentOS as our company uses that on it's managed servers.
It is predictable and easily configured and serviced.

I can't tell you how many times an "apt-get install x" has broken things.
Not everything in a Ubuntu repo works OOTB.

CentOS. It's free. It's solid and each release has a very long life.

I really like Debian Stable for servers. And it is quite good for desktop also.

I use Debian stable and Ubuntu LTS , Gentoo looks interesting with openrc init system on the server side

Even for server side of things, eh? And I'm not just thinking about running lts kernel here, but don't you think maybe a bit too bleeding edge? Yeah, rolling distros tend to find, and fix, things quickly, but maybe a bit too risky for applications where stability is key consideration? Seriously, I'm curious if anyone other than Arch project itself is really running this server side in a production environment?

Suse, Red hat, Debian

I am no expert but I have heard that some people prize Arch as a server OS. One reason being that the changes come in very small packages and are easy for sysadmins to adapt to, as opposed to a full reinstall with massive changes, another strength is that you quickly get security patches straight from the source code developers, this is a nice contrast to servers such as Debian and RHEL witch has to maintain essentialy legacy software.

But if you are planing to run Arch as a server you *NEED* to subscribe to the security newsfeed and always check it before doing system updates. It is also probably wise to exclude mission critical packages from your package manager, and update these manually.

Conclusion: It is possible to use Arch as a server, but I don't know if it would be "better" then the more traditional server distros, and I would not recommend it to lazy sysadmins :)

CentOS. I like RHEL systems and building from source. Yes sometimes I have more debugging to do, but I have more control and don't just go with they "okay install all the deps for me"

I only recently updated a Cent5 to the latest Cent6, everything was still working on the 5 box.