I really can't pick just one but I would say my top 4 are:
Lilo and Stitch: I love the whole family and Hawaiian theme, I love their culture, it is so beautiful. It is also my DD (5) fav. Disney movie. When she was 3 she used to put on a little hula skirt and dance around the house. She had the whole "stinkin tuna" monologue down pat and would recite it on a dime. I went CRAZY one Christmas trying to find all the Hasbro lilo and stitch dolls and playsets on ebay. ANother reason is ever since I was little it was "tradition" to go see the newest Disney movie when it came out, and this was the last one I saw "as a family" with my parents and sis.
Lady and the Tramp: I love dogs and I love the love story around them. And of course, my fav. scene is the Bella Notte spaghetti and meatballs part
Cinderella: I just love this story, makes me believe that in life anything is possible.
Nemo: I love under the sea life and the animation and references are wonderful! I also love Ellen and Dory and the Tank Gang, they are great
Oh and honorable mentions go to Up, first animated type of movie that as I was watching it I felt it was more of an "adult" story line, I cry like a baby every time Carl finds that adventure book that Ellie wrote in. And the Toy Stories, the concept, toys coming to life when no one is around, is great and I loved how all the toys I used to play with as a child had a personality hee hee.