AuronLu's FFX script: The Whole Shebang.
There's always more backstory to be mined, even tucked away in NPCs' comments. But who wants to go all the way back through the game to find them all?
An obsessive compulsive, that's who.
This is version 1.0 -- it needs spit 'n polish, and there are still a few gaps, like Clasko's alternate fate, what Luzzu says inside Djose temple before returing to Besaid, and NPC responses to Gatta's death or the Aurochs' losing.
Other than that, it's more complete than any script I've found.
P.S. I hereby challenge somebody to write a short fic explaining what happened to Keepa on the S.S. Winno.