The title has to go to Victor the Assassin from Tom Wood's series. It's not just his efficiency, but the psychological detachment that makes him terrifying. He's a tool, and the narration gets inside that void where moral calculations simply don't occur. It's chilling in a way that more emotional, brooding assassins aren't.
I tried reading some of the flashier names, but they often feel like action heroes with a specific code. Victor feels like a predator who happens to be human-shaped. The tradecraft details are what sell it for me—the meticulous planning, the weapon selection, the way he uses environments. It makes the fantasy disturbingly plausible.
Wood never lets you forget what Victor is, and that relentless focus is why he stands alone in my book.