While the fantasy genre (and to a lesser extent, science fiction) does have its share of good dragons, it's the evil ones that tend to be most memorable. Maybe it's because they display more intelligence in hatching their wicked schemes. Maybe it's because they've got bigger hoards (and, let's face it, when you're talking about piles of gold and other assorted treasure, size does matter). Maybe it's because they've got better dialogue (really, isn't it more satisfying to hear one roar "Puny scum!" than something soft like "Yes, I understand, that's a very good decision choosing lilac over key lime to paint your sitting room."). Or maybe it's sheer flashiness: there's nothing as visually stunning as a huge dragon wreaking havoc on its victims with blasts of fire, chomping of teeth, slashing and crushing of claws, and slamming of tail. The bad guys command respect.
For the purposes of this list, it's important to spell-out some disqualifications I've made.
Firstly, by "evil dragons", I mean dragons that are intelligent and have consciously made a decision that doing harm to other creatures is something they will pursue, with enjoyment, as a profession or hobby beyond their life-sustaining requirements. This rules-out dragons (whether they're intelligent or animalistic) like Vermithrax in Disney's Dragonslayer, or the monsters in Reign of Fire, or the hatching that grows out of control in Elizabeth Ann Scarborough's "The Dragon of Tollin" (in Greenberg & Yolen's anthology After the King) who devour and destroy simply as a matter of survival, and beyond that don't care about the affairs of other beings as long as they're left alone. This also includes the dragon in the original Old English poem "Beowulf", who seems indifferent to humans until a man steals a cup from his hoard, and then begins his rampage to get his property back and exact revenge. Eating and protecting one's territory isn't necessarily evil, though it may be unpleasant for those who get in the way of these top predators.
Secondly, I'm disqualifying dragons that aren't really dragons - ones that either started off as some other kind of creature, or ones where it isn't clear whether they're dragons taking the form of humans or humans taking the form of dragons for convenience. That rules out Fafnir of the Norse/Germanic legends (and it pained me to disqualify him, because Fafnir is clearly the inspiration for many of the dragons in the Western tradition, especially in modern fantasy) because originally he's a dwarf who runs off with a pile of treasure and sulks alone in the wilderness until his greed transforms him into a dragon. It also disqualifies the evil queen in Disney's Sleeping Beauty. I'm also leaving out the dragon in the recent animated movie Beowulf because he takes the form both of man and dragon, and being the son of a human and a demon, it's tough to know what his actual form is, or whether form matters to him. Likewise, Beowulf's dragon in Parke Godwin's fabulous novel The Tower of Beowulf starts off as a man transformed into a monster by greed, much like Fafnir.
So having divided our horde of dragons like different coins in a hoard, I give you
The Top 5 Evil Dragons:
5) Bryagh (from Gordon R Dickson's novel The Dragon and the George and the Rankin/Bass movie The Flight of Dragons)
In the original Dickson novel, Bryagh was more of a thug than anything. He began as the big bully in the flight that the good dragons, Gorbash and Smrgol, were members of and only really became evil at the end of the book when he was mustered by unseen dark powers to protect their fortress against the story's protagonists. In the movie, Bryagh (in the picture at the top of the post) is a real power to be reckoned with. True, the red wizard Ommadon is the emperor in the realm of evil, but the dragon Bryagh acts as his Darth Vader (Interesting, since Ommadon was voiced by Vader himself, James Earl Jones. Bryagh was voiced by James Gregory.), enforcing the sorcerer's rule but acting as a significant terror in his own right. Bryagh is dispatched to kidnap a man brought back in time to help save the world from evil, and when it looks like this might fail, the dragon is quite content to drop the man mid-flight in hopes that he'll crash and die. At the climax of the movie, Bryagh kills nearly all of the heroes, taking pleasure in taunting them as he smashes or roasts them, before he's eventually brought down. And if that's not enough, he even takes pleasure in eating the eggs of other dragons. This huge monster is wicked in the extreme.
4) Lien (from Naomi Novik's Temeraire novels)
Once a powerful member of the Chinese imperial court (in Novik's China, dragons are considered full citizens and equal to humans - unlike their Western counterparts, who are treated merely as clever livestock - and members of the rare Celestial breed like Lien are considered part of the royal family), Lien is intelligent, strong and driven. Perhaps her cruelty is partially due to the fact that she was always somewhat ostracized because she is an albino, her whiteness being the colour of death in Chinese superstition. But she also allied herself with a scheming lesser prince who had designs on the throne, and when his plot failed and he died, Lien went into exile and vowed revenge against the English-raised Celestial dragon Temeraire and his human companion Lawrence who had foiled the plan. But it's not enough for Lien simply to want Temeraire and Lawrence to die. She becomes so driven in her desire for revenge that she flies across the world to ally herself with Napoleon to help him conquer the world. By assisting the French emperor in his attempts to crush Britain, Lien hopes to make her nemeses suffer. Lending her own intelligence and Chinese learning to Napoleon's military, as well as own considerable strength, Lien helps the French air corps crush all opposition on the continent and secures a beachhead for an invasion force to land in Britain. She even uses her "divine wind" (an ability of the Celestial breed to generate a powerful sonic blast) to create a tidal wave to destroy many of Admiral Nelson's ships in a battle in the English Channel. That's a huge amount of overkill in pursuit of revenge, and certainly ranks Lien highly among the most evil dragons. What's more, that probably isn't the last from her. While Napoleon's army is forced off of Britain and Lien retreats, I think it's likely that she'll be back to cause more harm in some future novel.
3) Smaug (from JRR Tolkien's The Hobbit)
Going by nicknames like "the Golden" and "the Magnificent", Smaug is one of the great winged dragons of Middle Earth. He's also responsible for a huge amount of bloodshed. In taking over the Lonely Mountain, Smaug slaughtered most of an entire nation of dwarves, with the survivors being forced to flee to the homes of their cousins in the Iron Mountains or to live in the wilderness. He's also pillaged and killed in the elven kingdom of Mirkwood and in the human nations of Dale and the Long Lake. The fact that he's fairly old means he's probably guilty of a fair number of other atrocities farther back in history that weren't documented like the genocide described in The Hobbit, and so it's my guess that his crimes probably outweigh those of Lien. The only thing that put a stop to the dragon's depredations, and specifically his last angry rampage when Bilbo stole the Arkenstone from his hoard, was a good shot by Bard the Bowman, who managed to put an arrow through a weak spot in Smaug's armor.
2) Nomote (from Alan Dean Foster's "Lethal Perspective" in Greenberg & Greenberg's anthology Dragon Fantastic)
He may only be the size of a small hummingbird, but Nomote is responsible for causing suffering and death on a scale that most dragons can only dream of. Every so often throughout the ages, the last surviving dragons gather in a secret cavern in the Himalayas and brag about their wicked deeds. The one who has inflicted the most harm is given a golden chalice and named their leader. At the gathering in the current era, as the great wyrms go through the litany of disasters they've recently caused, they're disturbed when this tiny cousin enters and claims victory. At first they scoff. But all bow when he tells them that he's responsible for causing addiction and lighting a person's first cigarette. Sometimes it's the smallest fires that do the most damage to a village.
1) Glaurung (from JRR Tolkien's The Silmarillion)
It's almost not fair to include two of Tolkien's dragons in the top 5 of the entire genre, but he was just so good at creating them. The most evil of all the evil dragons I've encountered in books, TV or movies over the years is, without a doubt, Glaurung. In the history of Middle Earth, Glaurung is the Father of Dragons, the first dragon bred by the evil god Morgoth. In the great battles outside of Morgoth's fortress of Angband in the north in the First Age, Glaurung was an ultimate weapon, turning the tide of battle against the forces of men and elves until he was wounded by a dwarf king (who he managed to kill in his thrashing around, thereby forcing the dwarven army to withdraw to bury their leader, giving the forces of evil a chance to rally). Later he led a whole group of dragons in another battle, again driving the men and elves back. This wingless wyrm then led a goblin army out to destroy the elven city of Nargothrond, killing or enslaving everyone. But that's not enough: Glaurung likes to torment his prey from time to time. He puts a spell on the human warrior Turin, making him see the worst in himself. Glaurung holds Turin enchanted while goblins drag off the man's elvish girlfriend, then taunts him afterwards, telling him that if he tries to save her then he won't be able to save his mother or sister. The dragon then captures Turin's sister and casts a spell making her forget her identity and past; she eventually meets Turin, who doesn't recognize her when she's grown, and they marry. When Turin eventually meets Glaurung again and fatally wounds him, the dying dragon's last act of cruelty is to take the amnesia spell off of Turin's sister, causing her to realize with horror that she's married her brother; this drives the young woman to kill herself. A dragon that would use its dying breath just to mess with someone's mind one last time, to cause one last death that will make others suffer with grief, is truly the most evil of all. On this list of dragons, Glaurung takes the gold.