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.
I'm also a catholic (that's part of the reason I'm learning Latin), but I have to admit that my use of fiends is usually restricted to the Yugoloths and to the succubus/incubus. For me, the neutral evil fiends are the most evil variety, not really caring about law or chaos, but only about wickedness and power. Plus there's that whole thing about how you can control a Yugoloth if you know its true name, and that has a Solomon aspect to it that I find appealing. I also like how the Yugoloths are more prone to making Faustian bargains with mortals than Demons would be.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.