Saitama would win. He has shown incredible strength with just one punch. His power seems limitless, and in a fanfiction scenario, his simplicity and raw power would likely overpower Jiren's more complex abilities.
Well, it really depends on how the OC (Original Character) is designed. If the OC has some really unique and powerful abilities that can counter Saitama's strength, like the ability to nullify physical attacks or something really out - of - the - box, then the OC might stand a chance. But Saitama is known for his one - punch knockout power. So, in most cases, Saitama would probably win due to his overwhelming strength that has defeated almost every foe he's faced so far.
In my opinion, it's highly unlikely. Goku has mastered multiple forms and techniques that give him a wide range of powers. Saitama's one-punch style might not be effective against Goku's diverse skills and battle experience.
In a fanfiction battle, perhaps the fight begins with Naruto trying to talk to Goku first, to understand his intentions. Goku, not really understanding Naruto's world, just wants to test his strength. So he attacks without warning. Naruto quickly enters his Nine - Tails Chakra Mode to defend himself. The fight then escalates as Goku powers up further. The two exchange blows, with Goku's punches creating shockwaves and Naruto's attacks leaving marks on the ground. As the fight goes on, they both start to respect each other's power and skills, and it becomes more of a battle of wits as well as strength.
It depends on the rules set in the fan - fiction. If it's a battle based on strategy, Issei might have a chance. He has shown to be quite cunning in his battles. But if it's a pure power - based fight, Goku has the upper hand. His years of training under various masters and his Saiyan heritage give him an edge. Also, Goku has experience fighting opponents much stronger than himself and finding ways to overcome them.
I don't think there's a clear winner. Both Manga Goku and Manga Saitama have unique and powerful abilities. It could come down to strategy, luck, or even the author's imagination!
The battle between Saitama and Cosmic Garou in the fanfiction would have a catastrophic effect on the environment. First, Saitama's initial attacks might send out concussive forces that uproot trees and scatter them like matchsticks. Cosmic Garou's response could involve creating some sort of cosmic vortex that sucks in everything around it, from small pebbles to large boulders. As the fight progresses, the area around them would become a desolate wasteland, with fires raging from the heat generated by their attacks and the air filled with dust and debris, making it difficult for any living thing to survive in the vicinity.
Goku in Ultra Instinct mode is extremely powerful. He has superhuman speed, strength, and can dodge attacks effortlessly. However, Luffy also has his own set of amazing abilities like his Gear Fourth form which gives him enhanced speed and power. But I think Goku would win because his combat skills and power level are just on a different scale compared to Luffy.