https://www.fanfiction.net/u/9071421/ There aren't many good One Piece fanfics but try the few from my list or others. A good SI(self insert) that you're looking for is exceedingly rare, I wouldn't bother with them except for This Bites.
https: //www. fanfiction.net/s /12989970/1/Dial https: //www. fanfiction.net/s /12405840/1/Arcane-Chaos https: //www. fanfiction.net/s /12936816/1/Solaris-the-Solar-God https: //www. fanfiction.net/s /13078803/1/Fate-Hammer-Time https: //www. fanfiction.net/s /11515678/1/This-Bites https: //www. fanfiction.net/s /13187854/1/Friendly-Foreign-Dimensional-Exchange-Student-Spiderman https: //www. fanfiction.net/s /12713123/1/Assimilation https: //www. fanfiction.net/s /12085427/1/Displaced https: //www. fanfiction.net/s /12365777/1/Marvel-s-Maelstrom https: //www. fanfiction.net/s /10362076/1/Less-Than-Zero https: //www. fanfiction.net/s /13085438/1/Godhood-For-Dummies https: //www. fanfiction.net/s /12260666/1/A-Nobody
I've heard good things about this one! About a girl who transmigrates into the Naruto universe, WIP, 700k+ words, super popular. https://www.fanfiction.net/s/7347955/1/Dreaming-of-Sunshine
Yea it's good at first, but the problem is the author's probably a girl too, nothing wrong with that. Just that she overdid it with emotional drivel, a lot of people dropped it due to MC's whining. MC's brother Shikamaru gives her shit coz she's a girl getting into danger, and it's really soapy and tiresome.
If you like Nurarihyon no Mago anime, you can check out a fanfic in webnovel. It's called Nurarihyon System in Another World.