I honestly think searching for Nami/Luffy fics feels a bit different from other One Piece pairings. The dynamic isn't really about overt romance in the way you'd find for, say, ZoLu or Sanji/Nami stuff. So my list isn't about 'best' in a traditional sense, but more stories that capture their unique bond extremely well.
'Compass' by Woundup is my top rec for anyone trying the pairing. It's a canon-divergence from around Whole Cake Island, where Luffy picks up on Nami's lingering anxieties after everything. The prose has this quiet, attentive quality that mirrors how they operate on the ship—Luffy understanding her without needing a ton of words, Nami grounding him with practical care. It's subtle, no grand declarations, which makes the emotional moments hit harder.
For something completely different, 'Red Line Blues' is a modern AU where they're both struggling musicians. The author nails Nami's sharp-tongued pragmatism and Luffy's chaotic, magnetic energy in a totally new setting. Their chemistry feels authentic even without pirates or Devil Fruits. It's less about the epic adventure and more about two people finding a kind of freedom in each other's company, which is, at its core, very them.
I'd avoid anything that forces them into a typical romantic template too quickly. The good stuff lets their trust and partnership evolve naturally, often focusing on the crew as a family first. That foundation is what makes the rare moments of deeper affection actually meaningful.