Well, Don Bluth, Stu Krieger, Judy Freudberg and Tony Geiss are primarily the ones in charge in what happend in the film, itself, so perhaps the credit should be shared between them.
Don Bluth was the director, which gave him overall conplete control as, aside from making him shorten the film, Spielberg and Lucas seemed to have been relatively hands-off for most of the production.
Stu Krieger, Judy Freudberg, and Tony Geiss thought up the ideas, and Don Bluth, Gary Goldman, the actors, and the animators were the ones who had the make those ideas work. They also had to tell the writers where they needed rewrites when ideas weren't working, despite their efforts, and which ones flat out weren't going to work.
Overall, the creative process of the story was a shared responsibility for many people.