To learn the general principles of Windows Server operating systems, it is advisable to choose the most recent version. Today it is Windows Server 2019. If you're going to work in a specific organization (or with a specific server), you need to study the version of the system that is already used there. In practice, Windows Server versions are still actively used, which are no longer supported and updated by the developer (2003, 2008, 2012, 2016). Moving from an older version to a newer version is easier than vice versa, from a newer version to an old one. This is because newer versions always have a simpler interface and are more convenient to use.