CentOS 6, CloudLinux and Ubuntu 14
CentOS 7 or Ubuntu 14 are our recommended operating systems for all HTML, PHP and Flash websites, CMS and MySQL servers. They have active communities and are regularly updated.
CentOS 6 integrates best with cPanel and Plesk, so we only sell cPanel and Plesk servers with CentOS 6. If you are intending to run cPanel you should also consider CloudLinux which comes with extra features available within cPanel such as PHP Selector (enabling different websites to run different versions of PHP).
Other options include Debian and Fedora which are widely used.