Most fandoms have badfic outweigh the good. Look at MLP, my personal #1 fandom. It has a lot of garbage fic out there. Like, a lot, a lot of garbage. Like. Dang. Still, it is known as having some of the best fanfic writers out there. Why? Because we have a lot of really good ones. Dangerous Buisness, Past Sins, Fallout: Equestria, and on and on. We have more badfic, yes, but there is so much exceptional goodfic and plenty of general good or ok fic, it balances out.

Sonic, on the other hand, has no exceptional fic. Nothing that makes you say, "Wow. This was professional quality writing. I might even pay some money for this!" The best Sonic fanfic is just good. Nothing that really stands out. Just, OK. And that is why it has such a bad reputation; because without the brilliant, the horrid stands out all the more.

Well, two, but the other one has almost nothing, just the description.

One fic idea was following a kid who has never been chosen by their divine parent, watching as all these others get quests and gifts, and mostly, a sense of belonging. Then, one day, they are called before the Oracle. They have a quest to embark on. And yet, still no parentage. This is a journey not only across America, but of self discovery.

Neat, but, that is all I have.

As for the other... well, one day Scapegrace mentioned a Fallout crossover. And that got me thinking. How would Fallout go, where the Greek gods still around? I wrote up a war never changes thing for it, but I'm on mobile, so I can't get it. I'll post it when I get the chance. Suffice to say, I think it works out terrifyingly well.

Might be some here if you're interested.

http://tvtropes.org/pmwiki/pmwiki.php/FanficRecs/PercyJacksonAndTheOlympians

I'd say you could make the best out of this series if you've good knowledge and understanding of Greek mythology and stories.

And there was also that really weird DW/HP crossover where Harry became the Dalek Emperor...