It's a combination of many things. A being that has a vast array of powers, from telepathy to teleportation to super speed, like Flash in DC Comics, and also has a strong moral compass which allows them to use their powers effectively. And also, the influence they have in their fictional world. For example, Gandalf in The Lord of the Rings is not just powerful magically, but also has a great influence on the events and characters around him.
Immortality or near - immortality is important. Characters like Dracula in some fictional portrayals are very difficult to kill and have superhuman abilities. But it's not just about not dying. The ability to adapt and overcome any situation, such as Sherlock Holmes' mental acuity in solving the most complex cases which gives him an edge in his own fictional realm.
Power levels are a big factor. For example, if a character can destroy planets with ease like some of the powerful beings in science - fiction, that's a sign of great strength. Also, invulnerability plays a role. A being that can't be hurt easily is stronger than one that can be easily defeated.
I think it's a combination of different aspects. First, physical strength is important. Characters like the Hulk have incredible physical strength that allows them to overpower most opponents. Second, special abilities matter. A character like Scarlet Witch with her reality - warping powers is extremely formidable. And finally, the character's ability to grow and overcome challenges. A character who can constantly evolve like Naruto, from being a weak ninja to one of the most powerful, can be considered strong in the world of fiction.
Another factor is the character's ability to overcome any obstacle or enemy. A character who has no real weaknesses and can always find a way to win, regardless of the situation, could be seen as the strongest. This is somewhat like Batman. He doesn't have superpowers but his intelligence, strategic skills, and unwavering will allow him to defeat opponents much more powerful than him on many occasions.
Power levels are a big factor. If a character can do things like destroy planets or universes with ease, that's a sign of great strength. For example, Galactus in the Marvel Universe has the power to consume planets. His raw power is off the charts.
Well, in the world of Japanese anime and manga, Goku from Dragon Ball series is extremely powerful. His power levels keep increasing throughout the series. He can perform powerful energy attacks like the Kamehameha and has defeated many powerful foes. He has even achieved god - like forms which boost his already immense strength.
It's hard to say for sure. Different fictional universes have their own power scales and characters. But some might argue it's Superman or Goku.
Raw physical strength can be a factor. For example, characters like the Hulk in the Marvel universe have incredible strength that allows them to overpower most opponents. His strength increases as he gets angrier, making him a very powerful force.
The Death Note from the anime of the same name is a very strong weapon in the realm of fiction. It gives the user the power to kill anyone whose name they write in it, while also being able to specify the cause of death. This simple notebook has the power to cause mass chaos and reshape the world according to the user's will.
Power over the elements is a big factor. For example, if a being can control all the elements like fire, water, air, and earth, they would be very strong. In Percy Jackson's world, controlling water like Percy does is a great power, but if someone could do that plus control other elements, they'd be even more powerful.
Mastery of special powers or skills. In the case of Doctor Strange, his magical abilities make him one of the most powerful in the Marvel universe. He can manipulate time, space, and dimensions, which gives him an upper hand in any situation. So, a human in fiction can be the strongest through the possession of such extraordinary skills.