I would have to say Luke. He started his Jedi training at a late age, some thing the Jedi Order frown upon. What took most Jedi to learn in a life time only took Luke a few years to accomplish. The only Jedi that I know who purposely fell to the dark side and was fully redeemed.
IMO, I always looked at Luke as the Chosen One. Without Luke, Vader never would have killed the Emperor.