CentOS is still my favorite OS distro, but Debian is also nice.
It all depends what you will be using the server for I guess and what your preference is.
If you haven't worked with any of them, try CentOS.
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Debian has a better package management, and more packages in it's repository. If you want to do something odd with your server, Debian is way easier to work with. If you need a control panel, go CentOS.
I love debian, and all of my servers are Debian. It's really a preference thing, and it depends on the application.