In no particular order of course. Its hard to really sit down and rank these sort of things.
1. Lord Voldemort from the Harry Potter series: Lets be real here. He-Who-Must-Not-Be-Named was going to be on this list. Why would he NOT! He is terrifying, freaky, insane, and powerful. He brings the world of Harry Potter to its KNEES. Nevermind his attempts to avoid death and things. Even his memories are deadly.
2. Azash from The Elenium: A god. Specifically an Elder God. It took the 100 younger gods of Stryicum to trap him in a clay idol, and he still caused trouble for the world. One has to wonder if he was never found by Otha would he had caused all the strife that he had. In the end though, not even his godhood could save him. Also dude was a giant perv.
3. Goss and Subby from Kraken: China Meiville can do villains. And these 2 nut jobs are at the top of the bloody list. I have not been able to read much of Meiville’s work but the sheer terror these two inspire in Kraken is astounding. Goss is nearly impossible to kill, and Subby is just fucking creepy! Nevermind that the stuff that comes out of Goss’s mouth makes literally no sense, ever.
4. The Naagloshii from Turn Coat (Dresden Files): Where do I start with this thing. First, it eats magic users and gains their powers. Then, its a shapeshifter who can transform itself into anything it damn well wants. THEN, its bloody INTELLIGENT! It nearly kills Harry and several others and its only through sheer luck that they evade this thing. Oh and its seemingly immune to magic on its own.
5. Gaea from The Heroes of Olympus Series: How do you beat the literal spirit of the Earth? I mean honestly. Cause Gaea, when asleep, is deadly. Heaven forbid she ever wakes up!
6. The Lord Ruler from The Final Empire: What happens when Evil wins? Thats what The Fallen Empire talks about. In fact the entire book is all about how to take down a God Emperor who has rule for hundreads of years and is basically immortal. He also instituted horrible class systems to keep the working class on its knees.
7. Nicodemus from The Dresden Files: Yes a second character from the Dresden Files, but lets be real here. This guys is nuts. If the devil was a human, it would be Nicodemus. And it makes sense given that he is the head of the Knights of the Blackened Denarius, holder of the coin that houses the spirit of Andurial, a fallen angel. He is basically immortal as a noose he wears prevents him from dying. He is also highly intelligent and has lived for a LONG time.
8. Sauron from Lord of the Rings: Lets be real. One of the OG Badass villains is Sauron. Deceiver, Destroyer, Controller. Powerful, and nearly indestructible. He causes so many deaths and so much strife and corrupts so many people that he had to be here. The eye is watching.
9. Cluny the Scourge from Redwall: What can I say about this evil rat bastard. Other then he is really a Rat. No, really. A literal Rat. Redwall is a world depicting medievil life with anthropomorphic animals. And Cluny is the first villain of the first novel, Redwall. And he is a rat with a whiplike tail, who leads a horde of vicious vermin in an attempt to find..something. Because Cluny is not right in the head, at all.
10. Satan from Incarnations of Immortality: Literally the devil. But what made him interesting is that while he is the antagonist in the first 5 books…in book 6 we learn about how he came to be Satan. Cause like everything else in Incarnations of Immortality, Evil (the Devil) is just a job. And Satan does it with Style.
So there you have it. 10 Monsters or Villains from Fiction that I think are really awesome. Hope I did this right haha.