Not necessarily. There are many fictional characters with incredibly powerful abilities and it's hard to say for sure if Doom Slayer is the absolute strongest.
Well, the Doom Slayer's origin lies in a not - so - distant future. He was a part of the military stationed on Mars. The UAC's actions in exploring the unknown and trying to harness energy from other dimensions were the catalysts. They accidentally opened the gates to Hell. Our future marine was then thrust into a world of nightmares. Demons flooded in, and he was one of the few who didn't cower. Instead, he fought back with everything he had. His origin story is about a man pushed to the brink of insanity by the horrors of Hell but finding the strength within to become a slayer of demons. He has since been on an eternal quest to purge the universe of these demonic threats.
One of the key elements is his human origin. He was just a normal man before the disaster. The UAC's irresponsible scientific exploration that led to the connection with Hell is another important part. And of course, his decision to fight back against the demons instead of running away. This shows his courage and the birth of his new identity as the Doom Slayer. His experiences of seeing the destruction and death caused by the demons are what shaped his unending rage towards them.
The fanfiction might also play with the idea of redemption for the female demon. Since the Doom Slayer is often associated with justice, perhaps he sees some potential in her to change. It could be a story of her trying to break free from her demonic nature with the help of the Doom Slayer. Their relationship could develop from pure enmity to something more complex, like an unlikely alliance or even a romantic connection in a more extreme take on the story.
Well, 'Doom Slayer: Redemption' is a great fanfic. It shows the Slayer's struggle with his own inner demons while still fighting the literal demons. Then there's 'The Doom Slayer's Allies'. This fanfic explores the relationships the Slayer could have with other characters, whether they be human survivors or other powerful entities. And don't forget about 'Doom Slayer: In the Wasteland'. It's set in a post - apocalyptic world where the Slayer has to face not only demons but also the harsh environment.
Definitely the Emperor of Mankind. The contrast between the all - seeing, all - powerful (sort of) Emperor and the single - minded, brutally efficient Doom Slayer would be fascinating. They could have a complex relationship, with the Emperor trying to direct the Slayer's rage for the 'greater good' of the Warhammer universe.
One possible plot could be that the Doom Slayer is somehow transported to the world of Goblin Slayer. He then teams up with Goblin Slayer to take on an especially powerful horde of goblins that have been corrupted by demonic forces from Doom. Their combined skills in combat, the Doom Slayer's superhuman strength and arsenal, and Goblin Slayer's knowledge of goblin tactics would make for an epic battle.
There are definitely some 'Goblin Slayer x Doom' fanfics. One interesting aspect that some of these fanfics explore is the different combat styles of the two main characters. The Goblin Slayer is more methodical in his approach to dealing with goblins, while the Doomguy is all about brute force against demons. In a crossover fanfic, seeing how they would either learn from each other or clash because of their different styles can be really engaging. Also, the settings of the two can be blended in unique ways, like the dark and gritty world of 'Doom' with the medieval - ish setting of 'Goblin Slayer'.
Well, the plot could involve a rift opening between the two worlds. Goblins start pouring into the Doom world and the Doomguy has to deal with them. Meanwhile, the Goblin Slayer follows the goblins through the rift and they end up working together to close the rift and stop the chaos. They might exchange combat techniques and learn from each other's experiences along the way.
It could be a powerful demon lord from the Doom universe that has possessed a goblin king in the Goblin Slayer world. This combined entity would have the cunning and numbers of the goblins along with the raw power and dark magic of the demon lord. It would be a formidable foe for the Doom Slayer and Goblin Slayer to face.