Let's start with a simple question: what's your favorite fanfic? If you can't decide, maybe break it down: favorite one-shot, favorite long-form (100k+), etc. There's so many great stories in the fandom!
Stories by Northumbrian … start with “Tales of the Battle” and continue. His pre-Battle stories are not bad, but he really shines as a writer of post-war stories.
There are so many I like. That said, I think A Black Comedy would take the cake
It’s just a stupid, fun, ridiculous fic. I’m usually not into comedy types but it’s so good.
The best one-shot is, of course, Cauterize. If you haven't read it, do it now. It's only 1,648 words.
When I was first starting to read fanfiction I would have said my favorite was the Travel Secrets series. I haven't reread it in a long time because I think I'd probably be disappointed with the Super!Harry-ness of it, and the Slytherin PoliticsTM, though. I do still think it's a shame it was abandoned before being finished though.
Later when I decided my favorite ship was Harry/Tonks, I would have said Jamie Evans and Fate's Fool. I reread it recently, and still enjoy it, but the plot's kind of all over the place. Plus, it has horcrux Dumbledore. So that's another demerit.
Currently my favorite Honks fic is probably What's a Little Death Between Friends, which has a Super!Harry, but at least he's an adult post-war and iirc an Unspeakable, so it's easier for me to suspend my disbelief. Real shame it's only 3 chapters, even if they are beefy ones. Warning: this fic contains explicit sex between consenting adults.
Favorite fic I only read for the first time recently, even if it came out years ago, is The Sum of their Parts. I can't believe I somehow missed this one when it was new. I wish that the ending had taken it's time.
My favorite is A Year like None Other by Aspeninthesunlight. It’s an original story written before HBP came out, so it’s like an alternate take on what could have happened in those years. It’s very wholesome, Severitus. Some violence and self harm but it all works out. Laugh out loud and smile.