I just took my 2 year old and here are the movies she most enjoys and characters she liked the best. One caveat, she watches playhouse disney, so she LOVED seeing all of those characters at the studios.
1. The Little Mermaid (loved Voyage and went crazy meeting Ariel)
2. Sleeping Beauty (I don't totally get this one, but she loves it and really loved meeting Aurora)
3. Toy Story (any of the movies, there are just so many references at the parks, it helps them to see these characters on film first)
4. The Many Adventures of Winnie the Pooh (one of her favorites and is really good preparation for the ride, plus the characters are so cuddly. Mine does not like Eeyore though because he is just sooo big.)
5. Finding Nemo (again, there are so many references in the parks, this one really helps set the stage)
Honorable Mentions
Cinderella- loves this movie, loved meeting Cindy
Beauty and the Beast- no ride, but was mesmerized by the stage show because she was familiar with it
Peter Pan- she loves this ride and the movie both
Aladdin- she didn't make the connection between the carpets and the movie, but did enjoy seeing Jasmine and Aladdin
Snow White- still haven't taken her on this ride because I am worried it might scare her, but she likes meeting her