I'm surprised nobody has mentioned devils. Intelligent, evil, organized, manipulative, otherworldly, terrifying and preying on the flaws of humans. Maybe it's because I was raised Catholic, but devils have a feeling of being a ... deeper evil, more diabolical than even Lovecraftian horrors because they represent the evil of our own minds, amplified. Like, other bad guys are evil, but devils are Evil With a Capital E. Nothing seems better suited to the pervasive, overarching sinister plot that slowly builds over the course of an entire campaign - you can work in low level bad guys like humanoid cultists, summoning devils to foreshadow and gradually increase the power level, all with the sense of having that overwhelming, unknowable evil waiting just outside the gates of reality to pour in and turn everyone's life into a nightmarish Hell. I've built two campaigns completely around devils, and they usually find a way into most of my stories.