In addition to the favorites Will Hunting mentions, I also love Little Man Tate. I could so relate, and it touches on the humanity behind genius.
One site maintained by A.G. Reinhold is Math in the Movies. See also his linked page Mathematicians in the Movies, a webpage dedicated to films about mathematicians, including Feymenn, (also Einstein)! Included, e.g., is a film about Alan Turing: "Breaking the Code."
Reinhold's site also provides a link to a page listing feedback and suggestions he has received from others, Letters to Math in the Movies. For example, see Stand and Deliver. This is an inspiring movie! And it is based on a true story, directly relevant to math education (as it is about a high school mathematics class in urban LA, and the teacher that inspired them).
Consider, also Fermat, BBC's documentary on Prof. Wiles' proof of Fermat's last therorem.
Also N is a Number: A Portrait of Paul Erdős.
In terms of what/who I'd like to see a motion picture devoted to: I'd like to see a motion picture and/or documentary about Georg Cantor, whose work was not not well received by many of his time, but nonetheless, revolutionized mathematics.
Also, consider the work of Ian Stewart: Symmetry, The Mathematics of Life, and others could make for wonderful films, and Stewart has written a lot of fiction as well. Perhaps he might be encouraged to work with a screenwriter to develop a screenplay based on one or more of his books.
See The Story of Maths, a four-part documentary presented by Oxford professor Marcus du Sautoy.
A synopsis of the fourth episode, e.g.:
The fourth episode, To Infinity and Beyond, concludes the series. After exploring Georg Cantor’s work on infinity and Henri Poincaré’s work on chaos theory, he looks at how mathematics was itself thrown into chaos by the discoveries of Kurt Gödel, who showed that the unknowable is an integral part of maths, and Paul Cohen, who established that there were several different sorts of mathematics in which conflicting answers to the same question were possible. He concludes his journey by considering the great unsolved problems of mathematics today, including the Riemann Hypothesis, a conjecture about the distribution of prime numbers. A million dollar prize and a place in the history books await anyone who can prove Riemann’s theorem.