If you're asking who writes the most popular 'The Wild Robot' fanfiction, there's no single superstar author universally crowned across the web. The fandom for 'The Wild Robot' is comparatively small and fragmented, so popularity tends to be platform-specific: someone might be the top writer on Wattpad one month, while a creator on Archive of Our Own (AO3) or FanFiction.net has the most kudos or hits another month. I often suss out popularity by sorting by most hits, bookmarks, or comments, and by seeing which stories get shared in fan groups on Tumblr or Reddit.
What I love about this is that the most-read work usually reflects what that corner of the community wants — cozy expansions of Roz's life, robot/human crossovers, or whole alternate-universe retellings. You'll find lots of creators using handles instead of real names, so digging into comment threads and the author’s profile will tell you if they have a steady following. Fan artists and podcasters will also point to standout pieces; their reposts are a good signal.
If you want hard examples, the quickest route is to visit AO3, Wattpad, and FanFiction.net and sort by popularity within the 'The Wild Robot' tag, then follow the authors who consistently get high engagement. For me, that discovery process — reading a story that expands the quieter themes of the novel — is half the fun, and it often leads to finding a writer whose other works I end up loving.