I'm trying to learn to be a Unix/Linux Sysadmin, and am hoping to get some hands-on experience by setting up a few installs (VMs, old comps, and the like) at home and practicing administering them. I was wondering, in the experience of those on this site, which distros are critical to be comfortable with. Obviously, to a certain extent, they're all similar, but there are substantial enough differences between administering, say, Debian, OpenSolaris, FreeBSD, and CentOS.
So, which distros are most important to practice with in your opinion? Which have had most demand/come up most often in actual work situations?