For me, that would be Saving Mr. Banks. The way they handle the discussions in the creation of Mary Poppins I felt did a very good job and P.L. Travers and Walt Disney both have their reasons for what they wanted. All you gotta do is see it for yourself.
Now... seeing as I am the only one on the forums at this moment, I am not expecting any responses. But for those who happen sign up here, please be sure you don't give away any spoilers for your movie. Thanks.