Earlier quoted context omitted.
I think CentOS, Ubuntu and openSUSE, as specified in this page: http://training.linuxfoundation.org/ways-to-train/general-in... >The System Administration courses are written for CentOS 6, Ubuntu 12.04 LTS and OpenSUSE 12 (as the edX course should be the same as the course from the Linux Foundation, I'm assuming this applies as well).
While I understand the choice...I'm confident Debian is more popular than OpenSUSE at this point in history, at least in every market we work in (web hosting, mostly). We have tens of thousands of installations (or 1+ million, if you count Webmin, but we have much less OS data for that). OpenSUSE installs can be counted on a couple of hands. Historically it was pretty popular; SUSE may have even been the second most…
P.s. happy openSUSE user here :)