At the end of Season 14, we saw God/Chuck bring the hell souls back to earth. These are monsters that Sam and Dean had vanquished in the previous seasons before Supernatural's plot turned to focus more on angels. Throughout the years, the two brothers faced off against some truly imposing monsters, many of which gave the duo a real run for their money.
It's not easy hunting the Supernatural, especially when many of your foes are magical entities or have razor-sharp teeth. It begs the question, of those monsters and others faced in the seasons, especially the initial seasons, which were the scariest?
Changelings
Imagine looking in a mirror to see a child that looks like yours, but is noticeably different. At night, people's children are switched out by changelings, who then feed on the mothers until the mothers die. In Season 3, this happens to an old flame of Dean's, and the brothers save her and the real children, but not before there are some causalities, namely getting the fathers out of the way. The eeriness of the children sticks with you long after the episode is over.
Bloody Mary
One of the creepier episodes from Season 1, Bloody Mary comes when called, like the old story, but she doesn't necessarily attack the one in the mirror. She goes after the individual in the house with a secret, one that feels guilty. Bloody Mary slips from reflective surface to reflective surface, making her hard to escape from and hard to kill.
The guilty then see a reflection of themselves in the mirror, telling them of their guilt. Then their eyes bleed and eventually liquefy. It's not until Sam and Dean have the spirit confront her own reflection that she has to face her own crimes, which ultimately kills her.
Melanie Merchant: Little Girl Ghost in the Painting
Yet another scary and memorable monster happened in Season 1, another ghost. An old painting has a bad track record: Every person who purchases it, dies. When Dean and Sam torch the painting, it shows up miraculously whole the next day. Then they track down the body of the father in the family painting, the one they believe is the culprit of the killings. However, they soon find out that the culprit is the young adopted daughter.
She comes out of the painting, dragging her little doll behind her and holding an old fashioned razor in her hand. She was the one who killed the adopted family and her original family. In death, she kills those who possess the painting. Since her body was cremated, they are almost at a loss as to how to kill her. Then they realized that the doll (her doll) buried inside the mausoleum was made with her human hair. Luckily, Dean burns it in time to save Sam and Sarah, the daughter of the art gallery owner.
Hell Hounds
In later seasons, the hell hounds don't seem as scary, especially since Crowley makes them seem like his pets. However, in earlier seasons, the hell hounds terrify. Those that they go after do not escape. They drag Dean to hell. They kill and tear into Jo. We see Bela also damned to being dragged to hell by them. Part of their scariness centers on their invisibility.
Ferocious, single-minded, there is no reasoning or bargaining with a hell hound.
Scarecrow
In Season 1, Dean and Sam split up at one point: Sam to track down John, and Dean to the next monster. Dean heads to rural Indiana where out-of-town couples are disappearing. The out-of-town couples stumble upon this small town due to car problems. The town folk treat the out-of-town couples extra well by feeding them with good food, appearing to fatten them up. Supposedly, the cars are fixed, and the couples are given a specific route out of town. Really, they are given a route into the jaws of a Norse god brought by the town folk when they immigrated to the States generations ago.
The god calls for the sacrifice of one man and one woman once a year in exchange for good weather and crops. Desperate, the town folk choose to sacrifice one of their own, a young woman, and Dean. Thankfully, Sam comes in time to help because being chased by a hungry Scarecrow god is terrifying and hard to escape.
Yellow-Eyed Demon, Azazel
The demon that started the epic Supernatural story was Azazel, otherwise known as the yellow-eyed demon. Azazel killed Mary, Sam and Dean's mom, propelling their father into the life of a hunter. Later, he killed Jessica, Sam's girlfriend, to make Sam continue hunting, to become stronger. What was especially scary about Azazel is that he was a demon with a plan, and he was willing to work decades in order to put his plan in place. He created psychic children, human children fed by his demon blood. Then those children were put to fight against each other, Hunger Games style. The victor would lead the demons. Sam was his favored one. The amount Azazel terrorized these families and his persistence lands him here on this list. He was the most important demon and monster for the Winchesters for several seasons.
Angels: Alternative Universe Michael and Our Universe Lucifer
While our universe Michael isn't seen much and seems to take care of his vessels, alternative universe Michael was terrifying. He even appeared worse than Lucifer. This Michael wanted to take over the Winchester universe and create perfection: perfecting demons. He appeared unstoppable and invaded Dean, lying to Dean in order to stay in his preferred vessel. If it weren't for Jack sacrificing the last thread of his soul in the use of his magic to save the brothers and Cas, they would all be gone. Alternative universe Michael was like the Terminator, single-minded and unstoppable.
Lucifer is a foe, but occasionally becomes a temporary ally. Although funny and entertaining, he is quick to kill others and kills certain characters like Rowena in a horrific way. Lucifer is the reason that Rowena and Sam bond, both suffering from PTSD stemming from their encounters with Lucifer. In the end, Lucifer is even willing to use and kill his own son. We don't know if Chuck/God brought these two back, but if he did and they were to team up, it would seem that Team Winchester would have little chance to survive.
Zombie Crush
Season 2 introduces us to Sam's fear of clowns, and while we get a terrifying creature that morphs into a clown to gain children's trust, the zombie scares us more. Angela was a normal young woman, who upon learning that her boyfriend cheated on her, crashed her car and died. If that was the end of the story, it would've have been tragic, but not scary.
However, her best friend resurrects her as a zombie. Although she looks like Angela and has Angela's memories, she is a monster bent on revenge with a thirst for killing. All plants die around her. In the end, Sam and Dean pretend like they know how to kill her, but she is one of the only monsters that they can't kill. Instead, they lock her away in her grave. So while she wasn't a hell soul brought back by Chuck/God, she is still in the ground, rotting away and possibly thirsting for revenge.
Djinn
Our first encounter with a Djinn is in Season 2 when Dean is captured by one. The beauty and difficulty of the Djinn is that while it sucks your life away, it puts the person in a dream state where they get their heart's desire. Dean has always wanted a more normal life, and he dreams of one where he doesn't hunt.
The only problem is that Sam and he aren't close in this dream world. At one point, Dean wants to stay in the dream, even if it isn't real. For this ability and reason, the djinn are terrifying. They trap one in a web woven of their own hopes and dreams. When it is good or better than real life, the prey may not want to wake up.
Humans, the Benders
The worst monsters that Sam and Dean ever encountered were humans, a cannibal family that kidnaps people in order to hunt them. Even the young daughter is not redeemable.
In the episode, the Winchesters talk about how the demons they hunt have rules or a known behavior whereas these humans are worse. For being human, there is nothing humane about them and the way they treat others. The patriarch of the family talks about the great feeling of killing another human, and we can see by the number of photos, that they have killed many.
While we don't expect these monsters to be brought back in the last season, these humans were especially scary.
Soon, we will see which monsters make a long appearance in the final season. Most have a bone to pick with the Winchesters; hopefully, there will be allies to help Sam and Dean out.