I don't think I can rank both Disney and Pixar commingled into one list. They're too different. I can however provide a list for each production studio, separately.
1. Finding Nemo
2. The Incredibles / Wall-E
3. Wall-E / The Incredibles
4. Toy Story
5. Monsters INC
I haven't seen Ratatouille or Cars yet though, although I doubt either of them would knock any of the aforementioned out of the list. I guess the Rat has a small chance.
1. The Lion King
2. The Sword in the Stone
3. Fantasia (so ****ing underrated)
4. The Jungle Book
I'm actually just looking at a list right now of Disney animation, and a ton of movies are streaming back into my mind. Remember The Great Mouse Detective? Scary ass villain in that one, it was the big lumbering mouse with a back like the Hunchback of Notre Dame. Which reminds me - that was probably the first Disney movie I cried in. When all the villagers are throwing, like tomatoes and hotdogs and **** at the Hunchback. So sad. Pochahontas is another good one, I always forget about it. Can't forget Alice in Wonderland from back in the day, that's a classic. There's so frickin' many of them, I've got to start watching these things again. In fact, I think I might just go try to find Fantasia right now.