the villain that i do find interesting is, like many before me have said: Core Hunter.
His design and overall consept is the best of the HF villains and when you take into consideration that he was apparently going to have the "Skulls" of his victime around him (which you can clearly see the reason why that was droped) he is also the darkest villain from Hero Factory, almost like the Joker or the Carnage of this universe. A serial killer, who kills heroes and then as his calling card, takes their cores. Basicly if we where to go by real world standards or lets say the Bionicle standards, he is tearing out his victimes heart, and leaving them to die.
This is seriously how i imagine Core Hunter is removing a hero's core:
I actualy like the guy so much that i orderd him on Ebay. So he is my second Breakout villain, my first is XT4.