I think Light Yagami from Death Note could be an interesting choice for the most overpowered. He could use the Death Note to eliminate opponents in the game in a very sneaky way. For example, he could write the name of an opponent's character while they are in the middle of a battle, and they would just disappear. However, this would be a very different kind of overpowered compared to the physical strength of characters like Goku or Saitama.
Goku would probably be the most overpowered. His ability to constantly increase his power level and his wide range of energy attacks like the Kamehameha would make him a tough opponent in Super Smash Bros. He can also fly, which gives him an advantage in the game's arenas.
However, you can't discount characters like Kirby in this context. Kirby has the ability to copy the powers of his opponents in the Super Smash Bros games. If he were to face Naruto characters, he might be able to copy their jutsu and use it against them. Also, his floating ability and quick attacks make him a formidable foe. So, it really depends on how the author of the fanfic wants to balance the powers and abilities of the different characters.
It's likely that Naruto himself could be very powerful. His ability to use chakra and perform powerful jutsus like the Rasengan and Sage Mode would give him an edge. He could also use his shadow clones to confuse opponents.
One interesting storyline could be that Naruto, Goku and Luffy team up in the Super Smash Bros game. They use their unique abilities from their respective anime worlds to dominate the battles. Naruto could use his shadow clones to confuse opponents, Goku his energy blasts and Luffy his stretchy attacks.
It could be Naruto in his Nine - Tails Chakra Mode. He has immense power and speed, which would give him an edge in the Smash Bros battles.
Mario is often a very popular character in Smash Bros fanfic. He's such an iconic character from the gaming world that many writers like to put him in various scenarios, whether it's leading a team of Smash characters on a quest or having a one - on - one battle with a new villain in the fan - created story.
It's likely Goku. His Super Saiyan forms give him incredible power, speed, and energy attacks. He can constantly increase his power level and has a lot of combat experience from his battles in the Dragon Ball series.
Mario is often a very popular character. He's such a well - known icon from the gaming world, and in fan fiction, he can be involved in all kinds of adventures. He might be on a quest to rescue Princess Peach from a new villain in the Smash Bros world. Link is also popular. His sword - fighting skills and his mysterious backstory make him a great subject for fan - created stories. Writers can explore his connection to the Triforce and how it affects his role in the Smash Bros battles.
However, Saitama from One Punch Man (if we were to include him in this crossover fanfiction for the sake of comparison) would be extremely powerful. His one - punch ability means he can defeat almost any opponent with a single strike. But in the context of My Hero Academia characters only, Todoroki might also be a strong contender. His ability to control ice and fire gives him a great range of offensive and defensive capabilities in the battles.
Naruto and Kirby would be really interesting. Kirby could copy Naruto's abilities, like his running speed or his ability to use chakra. And Naruto could be amazed at Kirby's cuteness and his unique way of fighting, like inhaling enemies. They could have a really fun dynamic together.
Mario is often a popular choice. He's such an iconic character that people love to write about his adventures in the Smash Bros world. His relationship with other characters, like Luigi or Peach, can be explored in many ways.