Man, when I want the best Robin/Luffy stories I almost always start on Archive of Our Own. AO3's tagging system is a dream for hunting specific ships like 'Nico Robin/Monkey D. Luffy' or just plain 'Robin/Luffy', and I can filter by rating, complete works, word count, and whether it’s an AU, fluff, angsty, or smutty. I love that authors put content warnings and detailed tags—so you won’t accidentally dive into something you didn’t sign up for. I follow a handful of authors who write lovely, character-focused slices-of-life AUs for 'One Piece', and the kudos + bookmarks are actually good signals of quality.
FanFiction.net is where I find older, classic fanfics that shaped the fandom back in the day. The search is clunkier but there are gems—especially long, plotty fics and crossover experiments. Wattpad is great for newer, chatty authors who build series and interact in the comments; it skews younger and more serialized, so if you like following a fic update-by-update, that’s your playground. Tumblr and Twitter are where microfics, headcanons, and links to hidden gems show up—search tags like #robinluffy or #robinxlu.
A practical tip from my habit: use site searches (like site:archiveofourown.org "Robin" "Luffy") and follow rec lists on Reddit or in Discord servers dedicated to 'One Piece'—people curate their favorite longform romances and AUs. Also, bookmark authors you enjoy; fandom runs on follow-and-recommend, and that’s how I keep my personal reading queue full.