Debian

Debian is an OS which uses the Linux kernel. Though it is usually installed on personal computers, it is being used on more and more web servers too. The reason is that it's very stable, so a server running it will not have to be restarted for months. Debian is also regarded as being one of the most safe Operating Systems to choose from, as security patches are released within days after a problem is found. This is possible because thousands of well-experienced developers are working for the Debian project on a voluntary basis and not only do they work on the OS itself, in addition, they provide 24/7 support via special mailing lists and respond within 20 minutes to any question. Thus, you'll receive top-notch support directly from the developers who know their own platform in detail, and not from a third-party support team. Debian is also one of the Linux distributions with the highest number of packages - over 30,000. Since the Operating System is absolutely free to use, you're able to modify both the core and any of the packages in the way you see fit. Furthermore, you will not need to pay any kind of license fees.