Just scrolled through the whole thread and honestly, everyone's missing the obvious ones. The real classics that define the ship aren't just the fluff pieces—it's the ones that get the twisted dynamic right. 'Compulsive Hearts' on AO3 is basically required reading if you want that obsessive, destructive push-pull they had in the game. The author nails Nagito's internal monologue, how his luck cycles feed into his obsession with Hajime's 'hope,' and Hajime's frustration turning into a weird, protective fondness.
Then there's 'The Taste of Hope on Your Lips,' which is a much softer, post-canon fix-it. It handles the aftermath of everything, the healing, and actually makes a relationship between them feel earned instead of just slapping them together. It's slower, more about quiet moments, and the dialogue feels real. Avoid anything tagged 'major character death' unless you're ready to cry for a week—some of those fics hit way too hard.
Honestly, the ship thrives on the complexity. The best fics don't shy away from the mess.