There's an inherent, primordial viciousness to Professor James Moriarty that many of the names you'll see on this blog lack. Whether that's due to the cold-blooded, reptilian nature of the character himself, or to the circumstances of his creation is up for debate.
Moriarty was created to kill. Both literally, and figuratively. Created by Sir Arthur Conan Doyle in 1893, the good Professor was a literary bullet aimed at the fictional heart of Sherlock Holmes by his creator. There's a sort of sleekness about him, crafted as he was to be the dark mirror of Holmes, an insinuating sort of presence-we know who Moriarty is, because he is the reflection of Holmes. He possesses an implacable intelligence that we, as the reader, only understand as an afterthought. Deus ex Machina in a tailored suit and top-hat.
An evil genius with a criminal strain in his blood, his mental faculties rivalling or, indeed, exceeding those of Holmes and his brother, Mycroft, Moriarty was the emperor of the London underworld-a calculating Caligula, a soulless thing of cogs and gears and ruthless appetites. Where Sherlock Holmes is the World's Greatest Detective, Professor Moriarty is the World's Greatest Criminal. The perfect foil, the perfect executioner for a character that had taken on a life of his own, almost separate from that of his creator.
Too, for a character who lacked even the benefit of an on-stage canonical appearance (true, read The Final Problem...Moriarty only pops up in Holmes' hurried explanation to Watson), Moriarty has had an impact reminiscent of one of those heavenly bodies he supposedly theorized about in The Dynamics of an Asteroid. He's the phantom that haunts Conan Doyle's opera house, trailing his shadow across the life of Sherlock Holmes and causing it to wither, even after his primary purpose had been fulfilled.
After Holmes' miraculous return in The Adventure of the Empty House, he was never the same. Still the Great Detective, but somehow lessened, as if the memory of Moriarty's near-victory had a vampiric effect on Holmes' abilities. There are a number of further stories where the spectre of Moriarty haunts Holmes as he grapples with difficult cases, including His Last Bow, the final story in the official canon. The Professor even managed to worm his way retroactively into The Valley of Fear.
Then, consider the influence Moriarty has had on pop culture-eerily reminiscent of that of his enemy, dogging his footsteps beyond the boundaries of canon even as he trailed him across Europe. He has appeared in countless pastiches, sometimes facing his old foe, other times occupying the limelight alone (John Gardner's Moriarty novels, for instance). He has made appearances in films and various other media. He has locked horns with other malevolent figures, including Fu Manchu and Dracula (both of whom also fought Holmes and neither of whom came as close to destroying the Detective as Moriarty himself). Whatever media Holmes conquers, Moriarty is there, slinking through the back door, inflicting himself on the unsuspecting.
It could be said that Conan Doyle, desperate to kill one egregore, unleashed a second, even more sinister one. Desperate to eradicate the ultimate hero, he crafted the ultimate villain, and one thing that villains are good at is cheating death. Moriarty, created to kill Holmes and then fade away like a bad dream, clung stubbornly to life, thrusting himself into the public consciousness via the very act that was supposed to bring about his destruction. And like Holmes, he has flourished in the aftermath, albeit in a more subtle fashion.
Appropriate for such a shadowy figure, don't you think?