Arch Linux is an intimidating distro for the newcomer, but everyone should try it. The reason I mention it here is the documentation is phenominal for linux users in general: wiki.archlinux.org
Centos is a good distro to learn for server use, and most usefully Redhat's documentation is online - it is a the same lineage - with Kickstart being a good place to learn scripted deployment (versus imaging).
On the BSD side I've found FreeBSD beautifully simple to administer; package (port) management takes a little more learning though.
Biting the hand that feeds IT
