CentOS 4.x or 5.x
I use CentOS 5 as my dev server at home, and CentOS 4 as my live server (datacenter doesn't have 5 yet). At work we use FreeBSD on both dev and live servers.
With the right repositories (Atomic Rocket Turtle's) you can easily install php/mysql/apache/perl/python and other software.
That isn't to say that Ubuntu Server isn't good, or Fedora isn't good, it's just that CentOS is the free "version" of RHEL. So you get all the stability of RHEL in a free package. Other people choose to use Debian based distros (Ubuntu, Debian) and yet some others choose BSDs (FreeBSD, OpenBSD, etc). It's a matter of personal preference and just what you like in a linux flavor; however, at the server level, I think they're all pretty-much equal except for how you install things (apt-get, aptitude, yum, portage, etc.). Nothing wrong with any of them, just different software to get the same end result.