We have one production web, database, mail, dns and utility server that currently runs Debian. We also have a development / file server and various client servers that run Debian.
Debian was the clear choice for multiple reasons. Having started off my Linux background using Ubuntu, I wanted to use an OS that was similar but more suited for servers. I have used Debian/Ubuntu as my sole Linux choice for the past eight years. There are many reasons I continue using Debian, such as the package manager, the ample support and its stability.