Normally, I'd say Quick or Puckleberry fanfictions (although I amor some Finchel ones) but lately people have been coming out with some really impressive Sam/Kurt stories.
I'm a die hard Quick fan, so any of the Quick stories I find amazing! But I also find some amazing Brittana stories, which are usually hilarious and different, but very creative in my opinion. And then you read some totally aleatório never gonna happen ones such as Puck/Kurt, Quinn/Artie or something like Mercedes/Artie, which would never happen, but is fun to read!