It depends on the server and what it will be used for. If the server will house a particular software package, such as Oracle, ask the vendor what they recommend. What will be easiest for them to support? What OS has the best benchmarks for the software in your use case? Do you have staff who can work with that OS?
For generic servers, go with whatever your staff is most comfortable supporting. For basic web servers, if your staff knows their way around FreeBSD, use that. If they prefer CentOS, go with it. This will also somewhat depend on the software (you're probably not going to deploy Linux for an ASP.Net web site).
If there is a particular application well suited for a particular OS, such as a firewall, consider an OS with a reputation in that area. OpenBSD has a very good reputation for security, so they are an obvious choice to base firewalls on.