I feel the need to shamelessly pimp (and discuss) the most awesome story I've read, EVER. Though I doubt there are many people in the fandom left that don't know about it. XD
So yes,
Those Lacking Spines by Gexegee of Oganisation VI.
I have to say it was the fic that really got me into Organisation XIII, and - to be more specific - Vexen (... and Lex aaaand Xaldin). It's the reason why I love the pairings: 54, 34 and 534. It also made me realise many things about fanfiction in general - OOCness, Mary-Sues, the EMO-ness and total metamorphasis for the guys that usually end up playing the role of uke (Why do people do that?!). All whilst making me laugh so much it hurt.
I'm interested to hear what the fic has done for others, as well.
What were your favourite bits? What shocked/scared the living daylights out of you? Did anything surprise you? Did it make you look at fandom differently? Did you change because of it?