I run Gentoo, CentOS and Ubuntu on production servers currently.
Gentoo is good because you know exactly what is on your system and what isnt, but its also the worst for maintenance. Packages are bleeding edge (so you are vulnerable to 0-day exploits), must be compiled individually after every update etc etc. Any speed benefits are negated by the ridiculous amount of time you spend compiling things.
CentOS is demanding compared to the other two I run but it is the best documented and most stable. Being based on RHEL you know that all of the major packages are going to be there for it and that they will work. IME, it is the most secure straight out of the box too (I suppose with Gentoo it depends on what you install and what you don't etc).
Ubuntu is childsplay to install and get up and running with, apt is far and away the nicest package manager out there at the moment. Between this and CentOS, if you are a relative newbie I would choose this because you wont be using it in a production environment. You can transfer the skills you learn from here to the other, more complicated distros but you wont be overwhelmed with complex configuration choices from the get-go.