Honestly, I've seen this pairing blow up lately, but 'popular' can be such a weird metric on AO3. Does it mean most kudos, most comments, or just what's getting recced everywhere? I got sucked into this one called 'The Architect and the Wayfarer'—it's a slow-burn where Kaveh is trying to rebuild something in Sumeru and Lumine is just... passing through, but she keeps getting dragged into his projects. The author nails Kaveh's dramatic flair and Lumine's quiet exhaustion so well.
That said, the fic with the biggest numbers right now is probably 'Gilded Dust'. It's an AU where they're both researchers in the Akademiya, and the tension is through the roof. It's good, don't get me wrong, but sometimes I feel like the high-kudos fics are just the ones that update fastest, you know? I scrolled past a shorter one-shot, 'After the Rain at Pardis Dhyai', that absolutely wrecked me with its melancholy mood, and it has like a tenth of the engagement. Popular doesn't always mean the best fit for your mood.
I'd say filter by 'Completed' first to avoid the heartbreak of an abandoned WIP, then sort by bookmarks. That list tends to have more hidden quality than kudos alone.