I get a lot of insight from various teachers, most of all my sister, who is a 5th grade teacher in Kansas City, Missouri. I thought I would consolidate a bit of their advice on the top/most engaging movies for teaching. Feel free to add your own insights on the movies you use too!
Teaching Huckleberry Finn: Tom & Huck, 1995
Teaching The Prince and the Pauper: A Modern Twain Story: The Prince and the Pauper (with Disney stars Zac & Cody!), 2007
Teaching History: Horrible Histories
Teaching English: The tried and true Schoolhouse Rock
I'll add more as they are suggested to me!