@ P_206 / @ Baloon: Can't you both check what's this topic all about? The OP was implying what's the most suitable OS for server hosting and certainly he wasn't asking for a VPS provider.
Aside from this, everything which is focused towards servers hosting side, Linux is the way to go. Windows isn't that bad nevertheless although it hasn't crucial ups which Linux has. Before you're going to enter in the Linux environment, you've going to learn some basic commands and tenets of it. This tutorial helped me a lot when I was a novice onto this.
Having Linux and CentOS in the list doesn't make much sense because CentOS is a Linux distribution just like Ubuntu, Debian etc.
In general Linux is the most used OS for all kinds of servers, including game servers. It depends on your personal skills and preferences if a Windows or Linux server makes more sense. Both should work well with CS2D.
The easiest to use is, by far, Windows Server. I really recommend you to choose it over linux if the machine is near you, you don't want to go through a complicated process and you can access it directly.
Linux is pretty good when you can't directly access the server and is a dozen times lighter on both CPU and RAM than windows but it also quite more complicated and you'll need to go deep about it to properly use it and enjoy its features with no limitations.
I would pick linux, more precisely ubuntu server or arch linux, as I know how to work with it but if I didn't know how to use it and were not interested on it, I would pick Windows for sure.
Seeing as Linux is used for so many other applications, using it to host game servers would mean that you're not only getting the support of the combined millions of people doing the same, you're also learning how to use Linux using game servers as a practical example, and knowing how to use Linux systems is a very valuable skill indeed.
Ubuntu Server doesn't contain desktop environment, so if you are newbie in linux you should choose desktop version with build-in user-friendly graphical interface. Next time ask google about difference between server and desktop version. I'm pretty sure that there is a lot of similar questions.
@ Hajt: Ubuntu is Ubuntu, there is no actual "server" distribution or even difference apart from the pre-installed packages and setup. If you download the Ubuntu "server" version you can just use the following commands:
Download Ubuntu from the website then use https://unetbootin.github.io to create a bootable USB device. Follow their help page.
Reboot the PC then spam the key assigned to open your boot menu then boot the USB device from there. You should be greeted with a menu to install Ubuntu (there are other options like try, help or whatnot but just choose install).
@ _Yank: is right. You should have some previous knowledge before using Ubuntu Server. It has no GUI, so you need to know Shell commands. And even knowing Shell you will find a lot of problems so you have to know how to use Google to resolve them. With this thread it's clear that you don't know how to use it.