Critical (Unixlike) Distros for Sysadmin Hopefuls to Have Experience With [closed]

Debian, FreeBSD and CentOS.

They all have a pretty unique approach to how the system is laid out and particularly the way services are managed. If you can get your head around each and understand how to add to and modify the systems (in a way that not only works, but is congruent to the rest of the system) you will be able to work on anything.

Depending on the environment that you're looking to work in, you may find that Solaris is a useful skill to have on your CV too.

Redhat and distros based off of it seem to have the most penetration. Seems that every appliance I work with nowadays is using Centos. So if you really want to familiarize yourself with the most common distro, get Redhat or Centos (essentially the same thing).

Once you figure out how to be comfortable in that environment, familiarize yourself with a Debian based system. I'd recommend Ubuntu as it has a release cycle and maintenance periods that work well with corporations.