Honestly, I'm always a bit wary when people ask for the 'best'—taste varies so wildly, especially with OT3s. The Zoro x Luffy x Sanji dynamic can be tricky; it often ends up as Zoro/Sanji with Luffy as the oblivious third wheel, which isn't really the point for me. I look for fics where Luffy's perspective is central, where his unique way of seeing the world actually drives the relationship.
One that nailed that for me was 'Navigating Currents' on AO3. It’s a slow-burn, post-Wano story focusing on the three of them figuring out what they are to each other after everything. The author has a great handle on the crew's found family vibe, and the physical intimacy feels earned, not just tacked on. It's more about the emotional negotiation than the smut, which is refreshing. My only gripe is it hasn't updated in a few months.
Other than that, I tend to filter for the 'Romance' and 'Emotional Hurt/Comfort' tags and sort by kudos. You have to wade through a lot of PWP, but occasionally you strike gold with a character study that happens to include the three of them.