Why not start with their 70s stuff? "First Daze Here" is a perfect introduction and shows the variety of Liebling (and the other members') songwriting better than the full lengths.
It's interesting to see how they evolved and took on a more "metal" sound in the 80s, and I wish I'd heard their earlier stuff before "Relentless" and "Day of Reckoning." I still haven't heard anything newer than that... got a Sub-Basement LP lying around that I actually might wanna trade if anyone's interested -- just doesn't sound like it's on the same level. What can you expect from a guy Liebling's age?