One interesting feature of Linux is the freedom to create your own variation, usually called a distribution. Because of this, there are probably hundreds of Linux distributions, each with their own feature set, default packages, and general focus.
No particular distribution is considered clearly better than the others, although many Linux users will debate this point. For dedicated servers, there are definitely some that are better suited for a server environment, but that still leaves a large pool of available choices.
There are certain distributions that have been around for a long time, are well respected, have a large body of developers, and have organizations, either non-profit or for-profit, that support them. Such distributions include Debian, Red Hat Enterprise Linux, SUSE Linux Enterprise Server, Gentoo, Slackware, and many others.
There is a decisive advantage to using these types of distributions over ones that are new or that are maintained by a small group of part-time developers. When you are running a server, you need reliability. You need to know that your operating system will be supported for a guaranteed amount of time, even if it is a short time span. The last thing you need is to run updates on your system, only to discover your distribution has been abandoned.
Furthermore, the major distributions usually have the largest package repositories and have been tested by many other server administrators. If something goes wrong, you can rely on their expertise to find the problem. Ultimately, you have to make the best decision that is right for your particular situation, but choosing a major, well-supported distribution, is usually a good move.