Okay, so you're diving into the Percico rabbit hole? Buckle up. Honestly, I've been around this block a few times, and while the ship has had its moments, the absolute standouts for me are the ones that don't shy away from their inherent darkness and weirdness.
I'd point you straight to 'and the world, it shifts' by redacted. It's a canon-divergence where Hades claims Nico right after Bianca dies. It's less romance-focused initially and more about Nico's trauma and Percy's guilt, which makes the eventual connection feel earned and devastating. The prose is hauntingly beautiful without being flowery.
Another cornerstone is 'A Study in Resonance' by my dear imaginary author. It’s a soulmate AU, but the twist is the 'bond' manifests as hearing each other's thoughts during near-death experiences—which, for demigods, happens a lot. It manages to be funny, tense, and oddly tender. It nails their voices, especially Nico's dry sarcasm.
For something completely different, 'Let’s Cross Over' is a massive multi-fandom crossover that somehow makes Percico the emotional core. It’s chaotic and self-indulgent, but the character moments are golden. It’s the kind of thing you read when you want to see them interact in wildly off-brand scenarios and still feel like them. Those are my top three, hands down.