Depends on what exactly you mean by developed. The difference between A New Hope Luke Skywalker, in terms of personality and for lack of a better word, knowledge, and Fate of the Jedi Luke is pretty incredible. I might pick him.
Hmm, that's a hard one. Obi-Wan, Anakin, Luke are a few common names, a another one would be Kyle Katarn, he came a long way from the imperial soldier/agent to the Jedi Battlemaster of the NJO
__________________ "Happiness is a lie. Life is horror. The light is always dying all across the universe. The last star will flicker out someday, when it does, all that remains is shadow. And I will be its king!"'-Amahl Farouk
Though I'd put Corran Horn as more than him. Corran was the POV character for, what, 6-7 novels? And had significant role past that.
One thing about the big characters is while they had a lot of time, outside the main stories they weren't allowed to *develop* that much and often hit a development hamster wheel where they'd learn the same lessons a few times in a row. The most front time does not mean the most development. Comic fans, you know what I'm talking about- Dick Grayson/Nightwing has a dozen time the development of Batman/Bruce Wayne.
Second tier characters tend to get the most development. Big enough to get lots of time, small enough that it's ok to progress them in significant ways.
Oh, quite possibly Jaina Solo. Young Jedi Knights, Vong War, Legacy of the Force, Fate of the Jedi, she has a large chunk of her life and relationships detailed and she changes a lot during it. Possibly Jacen too, but there is the matter of his death giving somewhat less time, and his last arc sucking, but still, a lot of development.