In the mass of CL fanfictions(there are over 4200 on fanfiction.net alone, the last time I checked) catching the ones that are actually worth reading is like searching for a needle in a haystack. So, why not make things easier with some collective effort?
If you came across an interesting fanfic, which has breathtaking action scenes, has awesome character development, is funny, cute or - to the contrary- deals with difficult issues, overall, it really touched you as CL fans, is incredibly well written and you feel like sharing it with the others, DO IT.

It'd be nice if you could justify your choices and of course, inform about age rating. I'm not sure about M-rates, so I'd rather you didn't post them here. Mooods, am I right?
If a thread like that one yet exists, please correct me.
My favourites of all time:
The New Dawn T-rated. Basically, season 5. I'm a devoted follower of this story for over a year now and it still keeps on amusing me. It gives me chills every time I get a message about a new chapter, and I don't exaggerate

Jagged Edge T again. UxY oneshot angst turning into fluff at the end. Just... different from the regular pairing-centered fics.
Musical Exchange K+. I posted it in "Seeing Is Believing" thread, but I believe it's worth mentioning again. For development of a background character.
At the Lake K+. I wouldn't be myself if I did not mention J/A fluff. Straight from the master of forementioned pairing. As the title says.
The Wild Child part 2 part 3 and part 4! K+. Aelita as a faerie princess and Jeremie as an innocent villager. Wonderful AU fluff.
Wanna hear what you guys have to say.