Genos from One Punch Man himself would be great. He could be the one who tries to explain Saitama's power to the characters from the other crossover universe. For example, in a crossover with Attack on Titan, Genos could be the one to tell the Survey Corps about Saitama's invincibility as they face the Titans.
King could be a fun addition. In a crossover with Sword Art Online, King could be mistaken for a high - level player because of his reputation in the One Punch Man world. But in reality, he's just his normal self, and the SAO characters would gradually realize that there's more to him than meets the eye.
Well, first off, Captain America. His sense of honor and leadership could be a great influence on Spider - Man. Then there's Doctor Strange. He could introduce Spider - Man to the magical side of the Marvel universe. And of course, Deadpool. His wacky and unpredictable nature would add a lot of humor to the crossover. He and Spider - Man could have some hilarious banter while fighting bad guys.
Definitely Saitama. He's the main character and his nonchalant attitude towards his incredible power is always interesting to explore in fanfiction. Also, Genos. His loyalty to Saitama and his own journey to become stronger make him a great addition.
If we consider a crossover with the Lord of the Rings universe, Aragorn could be a good fit. He's a noble leader and a skilled warrior. His presence could force the Boys to reevaluate their ideas about power and leadership, and he could also be a strong ally against any corrupted power - hungry characters from the Boys' world.
The Terraria Guide could be a great character to include. He has a lot of knowledge about the Terraria world, so in a crossover, he could be the one trying to figure out the new world they're in. For example, if it's a crossover with Terraria and Hollow Knight, he could study the strange bugs and lore of the Hollow Knight world.
If it's a crossover with Teen Wolf, Stiles would be a great character to include. His quick thinking and sarcastic nature would contrast well with the more serious Shadowhunters. He could also provide some unique insights into solving problems, especially when it comes to the werewolf aspects of the Teen Wolf world that the Shadowhunters might not be as familiar with.
There's 'Saitama in Remnant' which is also a popular one. In this fanfic, Saitama finds himself in the RWBY world. His overpowered abilities create a lot of chaos and also some hilarious moments as he tries to understand the different rules and threats in Remnant. It also has some great fight scenes where his simple punch causes massive destruction in the RWBY setting.
Saitama is by far the most popular. His unique power of being able to defeat any opponent with one punch makes him a fascinating character to insert into other universes. Whether it's facing off against powerful foes in Naruto's ninja world or the superpowered individuals in My Hero Academia, his presence always shakes things up.
A really popular 'One Punch Man' fanfiction is 'King's True Power Revealed'. It delves into the mystery of King's character and gives him a more significant role. The story is full of twists and turns as it uncovers the secrets behind King's supposed power. Another interesting one is 'The Monster Association Reborn'. This fanfiction reimagines the Monster Association with new and more terrifying monsters, and Saitama has to face them in unexpected ways.
Well, Naruto's shadow clone jutsu could be really useful in a fight alongside Saitama. The clones could distract enemies while Saitama gets in a powerful punch. And Saitama's super strength could help Naruto in situations where brute force is needed more than ninjutsu. For example, if they had to break through a really thick and magically reinforced wall.
You could include characters like Harry Potter. His knowledge of magic from the Hogwarts world would contrast and interact interestingly with Louise's magic from 'zero no tsukaima'. Another good character could be Naruto. His determination and unique ninja skills would add a different dynamic to the story when combined with Saito's swordsmanship.