The obvious choice here is to go with Monty Burns, but Mr. Burns is evil in the same way that your great uncle who still refers to Vietnam as French Indochina is racist and politically incorrect. Yeah, technically, but he's just a harmless old guy so you have to cut him a little bit of slack. Sideshow Bob, on the other hand, is straight up psychotic. His attempts to capture and kill Bart are ridiculously menacing, especially for the likes of a cartoon that has a large following among children, and while Monty Burns is driven by greed, Sideshow Bob is driven by who the hell knows what. Which is significantly scarier. Is there are more malevolent, frightening character that has ever been featured on the show? Unlikely. And if The Simpsons' recent desperate play to lure in viewers is any indication, he might be getting the thing that he's longed for since his first appearance on the show.