Carnage isn't quite as nuanced, interesting, or sympathetic as many other villains on this list, but he does have something that most don't - pure, unadulterated bloodlust, which can make for some pretty terrifying villainy. Born of the combination of a psychopath serial killer and the offspring of the Venom symbiote (yes, symbiotes have kids, and their family dinners at Thanksgiving are total nightmares), Carnage was a essentially an absolutely destructive force of nature. Carnage was so evil and dangerous, he led to several Spider-Man/Venom team-ups to take down the gooier-looking symbiote. However, his weakness to really, really high-pitched noises has often proved his undoing, so as long as you keep a dog whistle around, you should be fine.
The following 8 entries must be pretty evil to beat out Galactus - the enormous, cosmic, planet-devouring space-god. He has no interest in conquering or simply destroying Earth - he wants to eat it. Our entire planet is nothing but a meal to Galactus. With his herald (such as the Silver Surfer) scouting potential planets for him, Galactus has eaten a fair chunk of the galaxy - even the homeworld of the Skrull. Sure, it's pretty evil to go around, munching on entire planets to fill your unimaginably huge stomach, but you can't blame the guy for trying to keep himself fed.
The Dark Phoenix is a destructive force, sure - I mean, it ate a freakin' star, destroying billions of lives - but the real evil of it is that it consumed a much beloved character as its host - Jean "Using My Powers Always Makes Me Faint" Grey - and continued using her body as its vessel. So we were stuck looking at a familiar character, but couldn't mourn her loss fully, because she was still right there in front of us. Eventually the Dark Phoenix was subdued, and Jean committed suicide (excuse me - apparently committed suicide, because "actually dying" simply isn't a thing in comic books for anyone not named "Uncle Ben").
Most would probably argue Bane shouldn't be this high on the list, but is experiencing a surge in popularity thanks to a major blockbuster movie that was released last year (Hotel Transylvania). Regardless, Bane managed to do something virtually no villain had done before - he took out Batman by breaking his back. It didn't require great cunning or expert planning, really - all it took was a hilariously 'roided up muscle-man in a luchador mask who was smart enough to wear Batman down a little before putting him against a foe way out of his weight class (which does take a decent amount of cunning and planning, to be fair). Still, he hasn't done much since sending Bruce Wayne to the chiropractor, but that act alone was a lot more effective than anything Calendar Man ever pulled off.
Ra's Al-Ghul is looking for a balanced, peaceful world - which he wants to achieve through killing most of the population, all while keeping himself essentially immortal utilizing Lazarus Pits (or, as they're commonly known, "plothole explainers"). What's nice about Ra's is that he and Batman have sort of a mutual respect for one another (Ra's refers to Batman as "Detective"), despite their very difficult methods to achieving their goals. They also share a relative, Bruce and Talia Al-Ghul's son, Damian. Something tells me that grandpa Ra's is usually left off the Christmas card list.