Double Agent Vader
The one where Vader turned double agent for the Rebellion.
Run a quick Google search of best Star Wars fanfic, and this one winds up on pretty much every list. There’s a reason for it, too. Imagine Vader — the Chosen One — actually embracing the light side of the Force while there’s still time to do something with it. It’s a really cool AU story that really shows how layered Vader is as a character.