Hi, MeLinda! This is a fantastic question.
As a Disney fanatic my very first thought is to watch all of them - every single one. These movies are fantastic! I see you are arriving in just a few short weeks (yay!) so pop some popcorn, snuggle under the blankets, and get comfy! I have put together a list of Disney animation for you that covers most Walt Disney World Resort attractions and character experiences.
The movies with the most presence in the Walt Disney World Resort Theme Parks are: Frozen, Beauty and the Beast, Peter Pan, The Little Mermaid, Cinderella, Snow White, Toy Story, Tangled, Aladdin, Finding Nemo, and Winnie the Pooh. Also it is a good idea to watch Chip 'n' Dale clips, anything with the 'Fab Five' (Mickey Mouse, Minnie Mouse, Goofy, Donald, and Pluto), and lots of Disney Jr. If you have time runners up would be Aristocrats, Mary Poppins, Alice in Wonderland, Jungle Book, Brave, Monsters Inc., Mulan, Sleeping Beauty, Pocahontas, Princess and the Frog, Cars, Lilo and Stitch, and The Lion King.
Wishing you and your family a magical vacation, MeLinda!
~Rachel