A three-part story "The Final Reckoning" by Ben Pieper:
A very long and epic fanfic with characters mostly from TNG, but also from DS9 and Voyager. The most important characters in the story are Picard and Q (even though the latter doesn't appear in many scenes). This is the best fanfic I've ever read.
The first part is about a devastating war spanning the entire galaxy, so expect massive amounts of character death. Character deaths actually make the first part depressing to read, but don't give up reading it. The first time reading I actually wondered whether the story would run out of characters.
The second part is the worst in my opinion. It's about a romance between Picard and an original character (there's a good reason for her being an OC).
The third part wraps up the events in the story. It has one of the best endings I've ever read in fiction.