Debian 3.0 (woody) is the first distribution I installed on a server and since than I fell in live with it. Debian was at the time the distribution with most packages provided in their repositories. Some of you maybe like to compile their own software, but for me software provided through distributions package management was always easier to maintain and upgrade and not to mention stable.

CentOS is also a nice distribution, but the version of Postfix shipped with the current release of Centos (5.6 at the time of writing) is ancient and is not really even supported by the author. Not to mention that the number of software packages provided through the official repositories does not even come close to Debians.

Software in the stable Debian distribution may be older than the one provided in Ubuntu, but I have more trust in Debian. Your milage may vary.