#1: ARTHUR MITCHELL, A.K.A. THE TRINITY KILLER (JOHN LITHGOW)
Season: 4 (12 episodes)
Killer bio: On the surface, Mitchell appears to be a good Christian family man who travels around the country constructing homes for the needy. In reality, he uses the concrete foundation of his homes to bury young boys alive, the first murder in a series of four which also includes bleeding out a young woman in a bathtub, forcing a married mother to jump to her death, and bludgeoning a father.
What he does to Dexter: When Dexter returns home after disposing of the Trinity Killer, he discovers that Mitchell had made Dexter's wife, Rita (Julie Benz), his final bathtub victim and left their infant son crying in a pool of her blood. We're not positive "last laugh" applies in such a sick, twisted situation, but if it does he got it.