The Marvel Cinematic Universe is filled with characters that have incredible powers, whether used for good or evil (or a bit of both). The Thor franchise in particular, with otherworldly gods and mythological roots, features a host of all-mighty and superhuman characters.
Thor: Ragnarok, arguably the most popular and financially successful of the group, is a veritable showcase of strength as the cast battles for the life and death of Asgard and its people. From Thor's electric level-up to Surtur's demonic fire, each character has unique attributes (magical or not) that make them powerful in their own right.
Grandmaster
Jeff Goldblum brings his usual unexplainable magnetism to the Grandmaster. He controls the entire planet of Sakaar and its people, treating it as his own hedonistic playground. He keeps the public sated with the extravagant Contest of Champions and seems to be able to execute anyone with the wave of a wand.
Grandmaster is a compelling villain. He is able, by extension of his leadership, to keep both Thor and Hulk as fighting slaves. Even though he doesn't have superhuman abilities or physical strength, his successful control and manipulation of the people of Sakaar prove he is a worthy opponent.
Valkyrie
Scrapper 142 first makes an appearance when she captures Thor, bringing him to the Grandmaster and collecting a bounty. She comes across as a callous and hardened individual who would rather drink and forget her origins and failures. She forms a friendship with Hulk and seems to enjoy training him, and eventually comes around to Thor's pleas for help saving their people.
Valkyrie is shown to have enhanced physical abilities, elite warrior training, and intelligence. She fearlessly leaps across moving spacecraft mid-flight, leaving a trail of broken vessels and ensuring she, Thor, and Bruce successfully escape to Asgard. As a Valkyrie, she wields a Dragonfang sword, which possesses incredible cutting abilities and gives her an advantage in battle. She and her Valkyrie sisters were unable to defeat Hela, but on the battlefield, she would be an incredible opponent for many of the other characters.
Odin
Facing the end of his life, Odin is no longer the strong warrior and king he once was. In Thor: Ragnarok, his Odinforce is fading which leaves an opening for new powers (namely, Hela) to take over.
Odin is the most powerful god in both the comics and the films. The fact that he was able to keep his daughter Hela, the Goddess of Death, confined for the duration of his life is a testament to the extent of his powers, but they aren't really evident in this installation of the series as he peacefully fades away early in the film.
Loki
Possessing both knowledge of magic and superhuman physical abilities, the god of mischief is a force to be reckoned with. Along with his enhanced abilities, He is incredibly charming, cunning, intelligent, and manipulative. Loki tricks the Asgardians into thinking he is Odin, and later in the film he infiltrates the inner circle of the Grandmaster, making his experience on Saatar much more comfortable than that of his brother.
Loki does not have the most powerful magic or superhuman strength, but his unpredictability gives him an advantage. Loki can be equal parts hero and villain, depending on how the scenario affects him personally, and relies on alliances and trickery to achieve his goals. Loki is a good match against his brother, but once Thor powers up that may no longer be the case.
Hulk
Hulk possesses superhuman strength and endless stamina. His biggest obstacle is his own emotions. In Thor: Ragnarok, he has greater control of himself. He brings his usual big green brute rage-induced strength throughout the film, including the incredible arena fight against Thor that proved to be a well-matched battle. Hulk is also the only one able to successfully take on Hela's fearsome wolf, Fenrir.
Although not explicitly shown, it is implied that during his time on Sakaar, Hulk racked up quite a body count of Champions. If he had stayed there forever instead of being rescued, he probably would have remained the reigning champion forever. With even greater control of himself, he could potentially be one of the most powerful beings in the universe.
Doctor Strange
Thor and Loki's first encounter with Doctor Strange gives a short glimpse at his expansive powers and shows that he is not simply a "second rate sorcerer" but a virtuoso of magic as a member of the Masters of the Mystic Arts. He is able to kidnap Loki, trapping him in a continual state of falling, and then transport both Loki and Thor through a portal to Odin in Norway with ease.
Even though his screen time is brief, Doctor Strange possesses an incredible mastery of magic and interdimensional travel. If he had to face-off against fellow sorcerer Loki, Doctor Strange would probably win.
Heimdall
Heimdall, as the eyes and ears of Asgard and the other realms, has unique and important powers. After being banished from his post at the Bifrost, he uses his sight to help keep the Asgardian people safe from (and to escape the wrath of) Hela, herding them into hiding and an eventual escape.
Heimdall is an important character in Thor's world. His position as keeper of the Bifrost and travel between worlds is powerful. His loyalty is to his people, and his bravery and strength alongside the second sight make him an important ally. He can see things other more physically powerful characters like Hulk cannot.
Surtur
Even though Thor successfully defeats the fire demon Surtur in the very first scene, once his power is restored with the Eternal Flame he becomes the only being who is a viable opponent for Hela. Covered in flames and wearing a crown of horns, he indeed does grow to the size of a mountain (as he mentions at the beginning of the movie) and brings about the end of Asgard by completely destroying it as well as Hela and himself.
The only being to defeat Surtur at his full strength prior to the events of the film is Odin. If Surtur were to be free with his full force, he would likely leave a path of destruction across the nine realms and beyond.
Thor
Thor, God of Thunder, is the firstborn (or so he thinks) child of Odin and a powerful god in his own right. The source of his powers, including energy blasts and storms, is his enchanted hammer. After the hammer is smashed, he learns that it is not the source of his power, but rather a way to control his abilities.
Thor is not the most powerful of the film characters solely based on abilities. He was unable to defeat Hela on his own, and a gladiator battle with Hulk was a fairly equal match. When he finally is able to channel his full strength and transforms into a walking ball of electricity he becomes a fully charged god. Embracing his thunderous potential sets Thor on track to be a great leader like his all-powerful father.
Hela
Odin identifies Hela as a threat right from the start. In the first five minutes of her appearance, she shatters Thor's hammer, Mjolnir, sends her brothers hurtling to a distant planet, and immediately gets to work taking over her ancestral home and the source of her power.
Defeating the Asgardian army, redecorating, and raising a dead army (including her wolf, Fenrir) with the help of the Eternal Flame were just the start of her reign of terror. She proved almost indestructible - the only way to defeat her was to obliterate the entire kingdom with her on it. It is highly unlikely any other character would have been able to beat Hela in battle.