As someone who is quite fond of all Hero Factory villains in general except for a select few lacklustre ones (Fire Lord, Scorpio, Voltix, and Scarox) this is a tough decision. I really enjoyed all the Breakout villains, which for the most part were phenomenal. I even grew to like a set I initially disliked, XT4, once I received it as a birthday present and had the figure in hand. I also, believe it or not, liked the Brain Attack sets as well.
However, If I had to pick one villain set in particular, It would probably be Witch Doctor. I remember seeing this set for the first time when pictures were first released of the summer 2011 wave of Hero Factory sets from TOYARK and the New York Toy Fair report of HF from BZPower back in February 2011. From the moment I laid my eyes on Witch Doctor, I instantly fell in love with him. That deliciously sinister face mould, his towering, imposing figure, the enormous, curved, wicked looking spikes protruding from his arms, everything about this set spoke to me. The only thing that disappointed me about this set was the perceived minuscule size of the skull staff he carried. While I would later accept the size of the staff once I actually obtained the set, at the time I was seriously upset. It seemed so underwhelming in comparison to the preliminary design of the staff visible on the preliminary box art displayed at the fair, which was far larger and more elaborate, adorned with long spikes and featured a black version of the prototype face mould mounted on the top of the staff. This was negligible in comparison to the remarkable design of the actual Witch Doctor model.
To this day, my Witch Doctor figure is still proudly displayed at the top shelf. We have yet to see another figure like him in mainstream constraction to this day, and for that reason, he must remain my favourite CCBS model of all time.
Though Core Hunter was an excellent set as well. He looks like a trophy hunter serial killer that stepped out of a slasher film.