Also, if for some reason, you have a desire to contribute to Ubuntu, contribute to Debian instead. Your changes will probably end up in Ubuntu, and the Debian folks care about stuff like user privacy and free software.
The realistic choice for servers today is either Debian or CentOS. This is what you'll find in pretty much any large deployment and for good reasons.
Ubuntu may be a distant third candidate due to its recent popularity, however it was never really meant for servers. Many of their policies are problematic in a server-env and most of the perceived popularity stems from inexperienced people who, by default, use on their server what they know from their desktop.
And please don't recommend SuSE. That is really just cruel.
I was on board the Fedora train until recently, but I had to get off because it was changing from RHEL beta edition to... RHEL pre-alpha edition. I have a soft spot for SuSE, but I've never been a full-time sysadmin, so you have to take my recommendations with a grain of salt.
At the same time, Canonical (makers of Ubuntu) are not very interested in the server, at least compared to Red Hat and SuSE. So you get big regressions in server-related functionality like the one described in this article.
Things not to use on a server: Ubuntu, Fedora, Gentoo Things to use on a server: CentOS, RHEL, SuSE, possibly Debian, any of the BSDs.