My introduction to BOC was the first album. A little ditty called "Cities On Flame With Rock And Roll" was what got my attention. The opening riff totally crushes, but is such a deft piece of playing at the same time. Then I got my hands on Tyranny & Mutation and "The Red And The Black" sealed the deal. As far as I'm concerned, every release through Spectres is well worth looking for and hearing.
I was fortunate enough to see them a few times during the early/mid 70's and they always put on an excellent show, definitely no going through the motions for them at that time. When they were really on, they were the musical equivalent of a Grand-Prix Formula One car - fast, loud and able to maneuver through the tightest of musical turns at breakneck speeds.