Carnage. I have his original three-issue appearance as well as the entire "Maximum Carnage" mini-series, along with several other Carnage-focused Marvel comics, and he's one of my favorite villains of all time. He's fascinating because he's presented as a character who's trying his hardest to be a two-dimensional villain; he kills simply because he loves it, and so puts himself up as beyond being explained or made sense of. In fact, he's someone who went through multiple traumas as a child (e.g. his mother tried to kill him, then he witnessed his father murder her for it, and he subsequently murdered his father), and is severely broken inside.
In trying to be a sort of archetypal evil, Carnage puts to the test to idealism of the superheroes who refuse to kill, making them question why they won't put down someone who sees a victory every time he takes a life and what it is they're fighting for. It presents some fairly deep moral and ethical questions, and helps to showcase the "heroes" in superheroes.