I don't have just one favorite, so i'll just list a few in no particular order..
The animation in this movie is stunning and unlike other 'silly' Pixar films, such as Monsters Inc, Incredibles and Cars, this one has more emotion and heart to it. The background music is also spectacular.
Hilarious film with some beautiful 2D animation. The songs are all excellent, and Robin Williams as the Genie is one of the funniest cartoon characters ever.
The Iron Giant
Originally, I thought this movie was going to be a dumb movie for 2nd Graders, but was pleasantly suprised. Next to Family Dog, I consider this to be Brad Bird's greatest work. Very funny, emotional, and some great animation.