Debian

As mentioned previously, Debian (the universal OS) is the basis for a lot of other distributions that came later, but it has consistently been one of the most popular, both on the desktop and on servers. It is still highly likely that you will choose to install Debian yourself, or will inherit a system running this distribution, with its reputation for stability.

Traditionally, the server-space war was fought between two camps, the Debian Druids and the CentOS Cardinals. In more recent years, newcomers have entered the fray (like Ubuntu,) but these two still hold a considerable amount of hardware to ransom. 

Releasing every two or three years, the Debian versions are named after Toy Story characters (7—Wheezy, 8—Jessie, 9—Stretch). They have a reputation for being one of the most stable Linux distributions around, with tried and tested versions of software, as well as sensibly backported fixes.

Some criticism is sometimes leveled at Debian because it generally has older versions of packages available in its release version, which may not include all the trendy and cool features a systems administrator wants, or a developer desires. This isn't fair, given that people generally look for stability and security in the server world, over the latest and greatest version of Node.js.

Debian has staunch defenders, and it holds a special place in a lot of hearts, though it's unusual to see it in some Enterprise environments because it's developed by the Debian Project, rather than a traditional company that can offer support contracts. In my anecdotally dismissible experience, I have more often seen Debian in smaller companies who needed a quick solution, and slightly larger companies who still run some legacy systems.