The Cuphead fandom has some truly wild creativity, and while there's no single 'most popular' fanfic, one that keeps popping up in discussions is 'Devil's Due' by MidnightStories. It takes the game's already chaotic energy and cranks it up to eleven, imagining what would happen if Cuphead and Mugman actually lost their souls to the Devil—but then outsmarted him in hell. The writing's snappy, full of that old-school cartoon vibe, and the author nails the brothers' dynamic. There's a ton of action, but also these surprisingly tender moments where their loyalty gets tested.
What I love is how it expands the game's lore without feeling forced—like giving the Devil a backstory that somehow makes him even scarier. The fic's been around for a while, but people still rec it because it captures the spirit of the game so well. Plus, there's a scene where Mugman uses a teacup as a weapon, and honestly? That's the kind of chaotic energy I live for.