For me it was Dennis Rader, better known as BTK:
He murdered 10 people and managed to evade the police for over three decades, mocking them with letters and nonsensical messages. He did not target marginalized classes like other killers; his first victims were an upper-middle-class family, the Otero family, in 1974.
Julie Otero, 38 years old.
Julie Otero, 34 years old.
Joseph Otero II, son. 9 years old.
Josephine Otero, daughter. 11 years old.
He gagged, tortured, and took the time to slowly suffocate each of the Otero family to death (Rader is a suffocation fetishist). Even the police, upon seeing the scene, thought they had been murdered by a professional hitman.
The worst part was that the guy wasn't one of those killers with a tragic past, abuse, rape, etc. He was just a bad guy who found pleasure in killing people and mocking the authorities.