I'll tell you what – newcomers should probably just stick with Archive of Our Own. It's not specifically for South Park, but the tagging system is a lifesaver. You can filter out the stuff you don't wanna see right away, which is crucial with a fandom this... creatively chaotic. I tried some of the dedicated sites years back, like South Park Archive or Fanfiction.net, and honestly, the organization feels like a 2005 webpage that never got an update. You're gonna wade through a lot of dated tags and weird crossovers before you find anything readable.
That said, if you're dead set on a dedicated space, the subreddit r/SouthParkFanfic can be useful for recommendations, but you're not reading on the site itself. People link stuff from AO3 there anyway. The real trick is learning to use the relationship and character tags on AO3 effectively. Want some classic Butters-centric hurt/comfort? Filter for it. Craving that specific rarepair from the later seasons? Someone's probably written it. The quality control is just higher because the kudos system surfaces the good stuff. I found my all-time favorite longfic, this insane deep-dive into Cartman's mom, just by sorting by kudos. You'd never stumble on that in a cluttered old directory.