Honestly, most people would point to the heavy duty angst fics, the ones with major character death or terminal illness, and those are powerful for sure. But I find the quieter, domestic hurts so much more gutting sometimes. There's this one I read a while back, I think it was called something like 'The Sound of Settling', and it was just about two of them living together years after the 'happy ending,' and the distance that grows from routine and unsaid things. The emotional theme wasn't a big tragic event, but the slow erosion of a connection you both swore would last forever. That's a different kind of heartbreak.
Another category that destroys me is the 'found family falling apart' trope, especially when it's from the perspective of the member who feels like they're watching it happen and can't stop it. The fics that explore the pressure of fame and how it strains their bonds—not in a dramatic, yelling-fight way, but in the way they stop sharing meals, or how conversations become scheduled and polite. You get this profound sense of grief for something that's still technically there, but isn't what it was. It's nostalgia and loss mixed together.
I'd also throw in the 'fix-it' fics that are emotional in a more hopeful, but still aching, way. The ones where they get a second chance after a fallout, but have to slowly rebuild trust. Every small gesture—making coffee for the other, remembering an old inside joke—carries so much weight. The emotional theme there is healing, but it's a bruised and careful kind of healing, which I think resonates deeply because it feels earned and real, not just magically solved.