Linux is a popular choice among operating systems. Deciding on Linux is however not enough. You have to pick a distribution and the list of choices can be overwhelming. Others have tackled the question and here are some answers:

An answer depends on the need. For me the primary need is for servers running SaaS applications and databases. In such scenario solid features, stability, reliability and longer term support are key requirements. Availability of professional support may be desirable as well. Distribution which consistently comes to the top of the list for such needs is CentOS/Red Hat Enterprise Linux. Ubuntu, with its Long Term Support version, is also a common choice.