Debian is an OS which uses the Linux kernel. Although it is usually installed on personal computers, it is being used on an increasing number of web servers too. The reason is that it is really stable, so a server running it won't need to be restarted for months. Debian is also regarded as being among the most safe Operating Systems nowadays, because security patches are released within days after an issue is discovered. This is possible because thousands of well-trained developers are employed by the Debian project on a voluntary basis and not only do they work on the OS itself, they also provide 24/7 support through special mailing lists and reply in 20 min to any question. Thus, you'll have top-level support directly from the developers who know their own platform in depth, 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. As the Operating System costs nothing to use, you'll be able to modify both the core and all of the packages in the way you see fit. Also, you won't be required to pay any kind of license fees.