It depends. Fedora core is what I'm running. But it gets outdated pretty fast and you'll need to upgrade.
Centos / Whitebox / RedHat EL are nice in that they get supported for 5 years or more, but they tend to feel old and creaky after a while. 🙂 RHEL is kinda the standard for hosting companies.
SUSE is good.
Ubuntu is very nice as a workstation OS, but as a server OS I don't really know.
So, if you wanna have a nice desktop that can double as a server OS, I'd recommend Ubuntu or Fedora Core as they're usually fairly up to date / modern. If it's gonna be a command line only server OS, then Centos is a good choice.