Okay, so 'best' is super subjective, but I've read a ton of Momoi x Kuroko stuff because I'm a sucker for that dynamic—the unrequited-but-is-it-really-unrequited tension from Momoi's side is just chef's kiss. I tend to avoid AUs that completely change their personalities, because the core tension for me comes from their established history and her persistence versus his obliviousness. A classic that nails this is 'Quiet Devotion' on AO3. It's a post-canon slice-of-life thing where Momoi is working as a data analyst and Kuroko is a teacher, and it's all about these tiny, accumulating moments where he starts to see her as more than just a manager from his high school days. The romance isn't in grand gestures; it's in him noticing she takes her coffee black now, or the way she organizes his bookshelf without being asked. Another one I keep going back to is 'Data Doesn't Lie,' which is a bit more canon-divergent and plays with the idea of Momoi using her data skills to try and 'calculate' how to win his affection, only to realize feelings aren't that formulaic. The pining is exquisite. I'd skip anything that makes Kuroko overly emotional or touchy-feely too fast, because half the appeal is his subtle shift from neutral to quietly reciprocating. Honestly, the tag is a bit thin on the ground for pure MxK, so I often end up browsing SakuRoku fics and hoping for good side-pairing content featuring them.
For something with a completely different vibe, 'The Spectrum of Pink' is a shorter, moodier piece that focuses on Momoi's jealousy—not of other girls, but of Kuroko's bond with Kagami. It reframes her longing through the lens of wanting to be his 'light' in the way Kagami is, and the romantic tension gets tangled up with this bitter-sweet professional admiration. It's less fluffy, more angsty, but the payoff feels earned. My advice is to filter by 'Momoi Satsuki/Kuroko Tetsuya' on AO3 and sort by kudos, but don't overlook some of the shorter, less polished works. Sometimes a 2k-word drabble capturing a single moment of eye contact or a shared umbrella has more romantic charge than a 50k epic.