As far as my choices go, as with cunningham1471 I find it quite difficult to narrow it down.
If we're talking in terms of actual animation then I think Bambi is absolutely timeless, and still looks gorgeous to this very day. The realistic and fluid animation of the deer and other animals must have looked amazing at the time, and the breakthroughs they achieved with incorporating real life special effects in the film such as smoke and water, multiplane shots and the enormous palette of colours generated were recycled in many other Disney films since. This film really did set the benchmark for animation, but it also had a mature enough story to suit all audiences and the idea of an unseen enemy makes Bambi worthy of a one-of-its-kind horror movie status.
Snow White and the Seven Dwarfs, Pinocchio and Sleeping Beauty are amongst Disney's best fairytale efforts. The Jungle Book has the best comical and musical value, although I think The Aristocats is an underrated comic gem.
A few films of the so-called Disney 'Rennaissance' were a bit mediocre IMO, namely Pocahontas, The Rescuers Down Under, Mulan and The Little Mermaid. Aladdin wouldn't have been as good if not for Robin Williams's inclusion and The Lion King is good and looks stunning, although in a glossy way.
I think Beauty and the Beast is my ultimate favourite, because it had the perfect "formula" that Disney films generally aim for and they ended up with a film that managed to be entirely enchanting, romantic, scary, uplifting, funny and sad throughout the duration. Two strong lead characters, good animation, only
one excellent use of CGI, great music, great voice cast, one of Disney's best antagonists and generally a great realization of a fairytale that was difficult to tackle and put across to their typical audience. Just how I went from feeling scared by the Beast in his first presence (I was six years old) to shedding tears when he gets heartbroken or when he died is testament to how well they handled it.
Live action would have to be Mary Poppins. Favourite Pixar film is Toy Story 2, with honourable mentions to WALL-E and Up.
In summary, my three favourites are:
1. Beauty and the Beast
2. Toy Story 2
3. Bambi