1.) Centos -> for cPanel users & some Plesk users
2.) Ubuntu -> for all open-source apps available on github + desktop users
3.) Debian -> for users who do not want want ubuntu
4.) Windows -> for users who agree to pay a license to use IIS
I have voted for CentOS because I find it more easy to manage in a server background. Though it is completely depending on the nature of use and the skillset of the admin. I think we cannot say there's a common good OS but depending on the environment and personals who's going to manage it.
CentOs is based on Red Hat backed by a big enterprise. The CentOs has gained much of popularity for serving as server due its security (which came from Red Hat) and totally free. So no doubt its the clear winner.
I preference CentOS. End of the day though, if you are a CentOS/RedHat Sysadmin, then you likely don't want to choose Ubuntu just because. Same goes for if your comfort and workflow is Debian/Ubuntu, then you might not want to choose RedHat/CentOS. Go with what you know. It always helps in keeping 100% uptime.
I've only used Windows, so I can't really know for certain if CentOS is better than Windows, but looking at the amount of votes and the things other people in this thread are saying, I suppose I should check it out. Maybe I should check it out.