Been scrounging around Archive of Our Own for a while now, and I keep coming back to a few crossovers that just nailed the weirdly specific energy of 'Zombie Land Saga.' There's one with 'Kamen Rider Ex-Aid' of all things, where the Franchouchou girls get sucked into the Bugster virus crisis. It sounds bonkers on paper, but the author totally gets it—the body horror of their undead state gets played for laughs in the show, but here it intersects with the more serious digital infection plot in a way that's strangely poignant? Like, Saki trying to comfort a sick kid by saying 'Hey, I died and came back too, it's not so bad' hits different when you remember she's literally a zombie. Another one that sticks with me is a crossover with 'The Disastrous Life of Saiki K.' where Saiki's psychic abilities keep glitching because he can't parse that the idols he's seeing aren't alive. It's mostly crack, but the author leans into the existential comedy of it all.
Honestly, I'm less sold on the super obvious crossovers, like with other idol anime. They often feel like they're just mashing two casts together without a premise. The ones that work best take the core contradiction—earnest idols who are also walking corpses—and throw it into a totally foreign universe to see how it breaks. There's a short, unfinished one with 'Cells at Work' where the girls are basically anomalous entities in the human body, and White Blood Cell is just perpetually confused and aggressive. It shouldn't work, but it's hilarious. I tend to filter by completion status because so many great ideas get abandoned after two chapters, which is the real tragedy of fanfiction.
I'd say the 'best' is subjective, but look for authors who aren't afraid to let the Franchouchou weirdness disrupt the crossover world's rules, not just have them sing and be cute. The humor and heart of ZLS comes from that dissonance. Also, maybe avoid the ultra-grimdark crossovers unless you're in that mood; seeing the girls deal with, like, the 'Attack on Titan' universe can be a bit too bleak and miss the show's balance.