Evil the Cat doesn't have the obvious "scary" factor that more traditional non-feline devils bring to the table. He's not even from Hell, because in Earthworm Jim he's the ruler of Planet Heck, which is probably where evil Canadians and great-aunts face perdition. Still, Evil the Cat is one of the hardest boss Satans to beat. Evil forces you to fight your way through levels of fire and lava and "Night on Bald Mountain" / elevator muzak. Then when you finally face him, he's stolen your supersuit, leaving you a vulnerable, pathetic earthworm. Once you get it back, you still have to fight him off as a set of evil yellow eyes coming at you out of nowhere. And don't ignore the next-level creep factor here: it's a version of Satan named Evil The Cat, and it's horrifyingly...plausible? Maybe your pet kitty doesn't like you that much after all, even though you went to the trouble of naming him Meow-ford And Sons and buying him a cat blazer and everything.
To ancient Greeks, Hades was not as we know him in the modern world (the embodiment of evil as voiced by James Woods). Instead he was a patron god of plutocrats and a necessary handler of the dead.
So it's a good thing videogames came along and made him a little more interesting, because nobody wants a nuanced consideration of the Circle of Life when you can take on this spiky freak. He's got Claws of Hades to tear away the soul of any living being, be they mortal or Titan. He can summon hosts of the undead to fight for him. When Kratos beats the crap out of him, he claims to enjoy the pain - and don't forget that the souls of those he kills make him stronger. The videogame version of Hades is straight out of the bloody, depraved version of ancient history, and he's so terrifying that even Kratos, who goes around killing deities professionally, finds him particularly challenging and horrifying. And that's coming from the guy who tore a fingernail off a god that was 5,000 times bigger than he was.
Dante Alighieri was a Florentine political hanger-on who wrote a novel symbolically torturing people he couldn't beat up in real life. Now look at the real poet, and then look at the hellblazing badass the videogame Dante's Inferno makes him into:
Since the Dante's Inferno designers made that literary prick into a knight who goes ham against Death itself, naturally they made the game's villain Lucifer go from "already horrifying" to "AAAAAAAGH NOOOOOOO SHIIIIIIIIIIT, hang on what? No, Mom, I'm fine, I'm just playing PS3."
Frozen in a lake of his own tears, their Lucifer appears as a spectral shadow anywhere he wants, who forces you to flashback to when you were committing war crimes in the Crusades, and drags your wife Beatrice to Hell to marry him. The boss battle's another solid top-demon affair, layered over with a hint of the literary and spiritual significance you only survived in high school with the saving power of SparkNotes.
The Prime Evil of all three Diablo games, Diablo is (to summarize a lot of tortured in-game mythology) the baddest devil of them all. And the chance to kill him feels truly earned, because you'll only come face to face with Diablo's title hellspawn once you've hacked and slashed your way through thousands upon thousands of waves of enemies, and level-grinded your way to the apex of several special abilities, and learned a Runic language so well you know it better than the periodic table. Heads up to Diablo III players who aren't done yet: get ready for a long-awaited boss battle that will involve a lot of running around beneath a colossal, horned, ugly sumbitch who raises spikes from the ground, shoots fire in all directions, and generally acts like the guy your momma warned you about (if your momma was a fire-and-brimstone Puritan preacher).
Also, the always-on DRM must be the invention of Diablo, right?
As part of a series packed with devils of elaborate, creative varieties, Doom II: Hell on Earth has the cyberdemons and mancubi you could want. But it tops itself with the Icon of Sin, a billboard-sized monstrosity whose gruesome death closes that portal to Hell once and for all (sort of). Devils are scary enough, but having to fire rockets into the exposed brain of a demon-whale until it splatters itself across miles of ruined Earth? That's some almost inconceivable terror. Inconceivable terror to most, anyway...somehow Doomguy just keeps quizzically raising his eyebrows and looking around to the sides. HE'S (peripherally) SEEN SOME SHIT.