Debian is a well-known Linux distribution, that's used primarily on desktop machines, but many people have started using it lately as a web server OS as well, due to the stability and protection it supplies. The latter is a consequence of the fact that Debian is developed and supported by thousands of qualified volunteers internationally, which means that any patch which is needed to fix a certain issue is released within a couple of days. The same community will also help with any question or trouble that a user may ever experience, since they can be contacted through e-mail using mailing lists and a competent response will be sent in no more than 15-20 minutes. The advantage here is that all your queries will be addressed by the same people who have designed the Operating System itself and they have a detailed knowledge of how it works. As Debian is free, you won't have to pay any license fees to use it and you will be able to personalize it as much as you want or need. This is valid for both the Operating System as well as the 30.000+ software packages which are readily available for it.