Ubuntu Server

As I mentioned on the File Server page, this is the de-facto server for anyone wanting to host services.  It's stripped down to only the most necessary apps for the purpose of remote connecting and setting things up.  It's free and it has great community support.  There's not much more you can ask for, so long as you know what you're doing.

It's not the best for beginners because it doesn't hold your hand or even provide a web UI.  You can install your own web UI and gain all of the conveniences of any other server OS through various apps.  There's not much of a limit to what you can run on it.

This is what I call the baseline for any server.  Everything else is a trade off of advantages and limitations.