"I won" (Season Four, Episode Thirteen: "Face Off")
We've seen how low Walt can go. But how far? Poisoning-innocent-children far, as we learn in a season-ending revelation that turned Breaking Bad's "hero" into its villain in just six words: "Lilly of the Valley" and "I won." Presented throughout the course of the season with a variety of ways to get out of his one-man war with Gus, Walt chose the one that would prove to them both that he, not the Chicken Man, is the smartest guy in the room. If that means nearly killing Jesse's girlfriend's kid in a last-ditch, so-crazy-it-just-might-work attempt to convince Pinkman it's all a master plan by Gus to drive a wedge between them, so be it. Until now, everyone in Walt's crosshairs had threatened him in some way; Brock was an innocent bystander, but Walt didn't care. It's his clearest contrast yet with Jesse, who, ever since his encounter with the poor neglected redheaded kid in the meth-heads' flophouse back in Season Two, has demonstrated his concern for children time and time again. Even if you believe, as I do, that chemistry-expert Walt calculated a dosage of the poison he knew to be non-lethal, it's not like anyone told that to Brock or his family. Their physical and emotional anguish is real, to them at least. To Walt, they're just pawns in the meth game of thrones. The king is dead. All hail the king.