Most 'Sonic Prime' reader inserts I've stumbled onto lately seem to congregate on Archive of Our Own and Wattpad, but the quality spectrum is wild. AO3's tagging system is a lifesaver for filtering out what you don't want—you can really drill down to 'Sonic Prime', your preferred version of Sonic, and avoid the characters you're not vibing with. I've found some genuinely inventive takes there that play with the 'Shatterverse' concept, letting the reader interact with different versions like Nine or Rebel. The prose tends to be a bit more polished on average, though you still have to wade through the 'y/n' stuff which I know isn't for everyone.
Wattpad feels like the wilder frontier. The algorithms there push popularity, so the top stories often have that very specific, fast-paced, dialogue-heavy style that resonates with a younger crowd. You'll find a ton of shorter, fluffy oneshots and dramatic multi-chapter adventures where the reader character gets thrown into the action. Discovery is rougher, and you might click on a story tagged for Shadow that only has him appear for one paragraph. Still, the sheer volume means there's always something new, and the comment culture is very immediate, which some writers thrive on. For this particular fandom, I'd probably start with AO3 and use Wattpad as a secondary dive when I've exhausted the better-tagged options.