Debian and Red Hat are great to know. Once you understand rpm and deb package management you can fix broken yum and apt get problems. After you get that down you have masted 90% of the Linux world.
Then when your serious download Slackware and get to work. Build from source and control where your apps live. Much more enjoyable than spec files.
http://www.slackware.com
Backtrack did a version based on slax ( which is based on slack ) a few revisions back.
From auditor and whax to backtrack to Kali. What's next?