There really is no such thing as a "server OS" (at least not in our market segment). There are certainly package configurations and types of software that make a machine better suited as a server, but there isn't actually anything different about the core operating system. With this in mind, we really need to consider what we want in a "server OS".
For the purposes one would use a Raspberry Pi for, I think we could sum it up with two main points. In the "real world" there are other points, but they don't really apply to our use case.
The server should be easy to maintain
There shouldn't be a lot of junk installed. This uses extra disk space, bandwidth, and upgrade time. Things like a desktop and Mathematica aren't really needed, so why have them?
In my opinion, Raspbian covers the first bullet really well. Looking at it from the command line, it's very similar to Ubuntu Server. Both are Debian based after all. From the second point... To put it nicely, the default Raspbian distribution is rather bloated.
@Ghanima mentioned Arch, but for most people, that meets the second point, but widely misses the mark for point one. Some people love it, but a large percentage of people find it rather challenging.
My recommendation is to use Raspbian Lite. Just looking at the image shows a much smaller size, and everything you want to install is just an
apt-get call away.