Although he's not the main character, one of the most potent forces of power in Hajime Isayama's Attack on Titan is easily Levi Ackerman, who steals the show time and time again with his ruthless and calculated fighting skills paired with his intense demeanor. However, it's more than just Levi's skills and personality that make him so powerful. As revealed in Part 1 of the final season, Levi has a secret power that makes him the most formidable and powerful character in the entire anime.
Because Levi is so profoundly strong on multiple levels, many have theorized that he could be a Titan Shifter. Though this theory has been proven wrong, the truth isn't too far off; created from a similar method to the Shifter Titans, the Ackerman clan are a bloodline of warriors that originally protected Eldia's king. They were born from experiments with the Subjects of Ymir and Titan science, producing a family of humans stronger than the rest, bred to protect the most important person.
This human-hybrid clan of Ackermans was gifted with an ability that aids them with their ultimate protective purposes -- this "awakened power" allows them to know exactly what needs to be done, according to Levi's explanation. On top of Levi's already heightened intelligence and analysis skills, he has the ability to know what needs to be done to win in any given situation. This means that even up against the Titans, Levi will always succeed. Add this power to his long list of skills and physical prowess, and he becomes the most overpowered character in the series.
Like his relative Mikasa Ackerman, Levi will always have the upper hand in a physical battle. That's a guaranteed fact due to their shared awakened power. However, Levi suffers a traumatic injury at the end of Season 4, Part 1, which will hinder his physical abilities in the future. Nonetheless, even with this injury holding him back, Levi still has the potential to be the most dangerous person around.
If fans are ever gifted with a sequel, it's not too far-fetched to believe that Levi could very well become a villain. He would be unable to fully act on his natural instincts anymore, which would create a keen sense of frustration in anyone. This emotional turbulence, combined with his known luck to always be where the action is, would create a version of Levi that takes more dangerous risks. While it's hard to view Levi as a villain in any context given his righteous attitude, he certainly has the capabilities of one.
Typically, a villain is cold-hearted, intelligent and analytical, with strong physical abilities to boot. Levi hits all of these benchmarks with flying colors, even outranking some of the actual villains in Attack on Titan. He doesn't even really see Titans as a threat anymore, considering how easily he takes them down on his own. While Eren Yeager is the most aggressive and unyielding fighter in the series, he is still no match for Levi -- not even close. Levi is one traumatic experience away from becoming the world's most dangerous villain.
Thankfully, Levi is simply too righteous and honest to go bad. His incredible skills and awakened power have always been put to the use of good and with the hope of peace. Even though he's instinctively violent and quick to act, Levi has thousands of years of genetic power built up inside of him, so it's easy to understand why he's like this. One thing's for sure: Eren Yeager is not the most powerful character in Attack on Titan -- it's Levi Ackerman.