CentOS, CloudLinux and Ubuntu
CentOS or Ubuntu 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 integrates with cPanel and Plesk. If you are intending to run cPanel you can also consider CloudLinux which comes with extra features available within cPanel such as the LVE manager to separate resources between websites.
Other options include Debian which is widely used.