CentOS is great server operating system. If You are RHCE then You will be very familiar with it because it is based on RedHat Enterprise Linux source and is 100% compatible with it.
If system is mission critical then usually use commercial supported OS - SUSE Linux Enterprise Server or RedHat Enterprise Linux. Commercial Linux will provide faster updates and support will help to solve problems.
Ubuntu is interesting player in market and after some time could be very strong. They doesn't separate commercial and community/free versions (RedHat has Fedora, SUSE has OpenSUSE). If You need support - just start paying. Problem is that it haven't software and hardware certification in required level.
For example, if You will buy HP server then You can be sure that it would work with RedHat and Novell SUSE flawlessly. Lot of commercial software do the same (for example Lotus Domino/Notes) Ubuntu Server haven't such advantage yet.
I'm not expert in Linux desktop market (my desktops are running winXP, sorry
), but Ubuntu is looking better than RedHat and Novell there.