I guess I'm in the minority - A Bug's Life is my favorite one. It's probably not the best Pixar movie, but it makes no attempts to be a "deep" movie, unlike all of the rest of them. I enjoy it for being fun, no-holds-barred, piece of pure entertainment. I can't remember a more thrilling moment in a Pixar film than the moment the rain appeared towards the end of A Bug's Life.
A Bug's Life > The Incredibles > Toy Story > Toy Story 2 > Finding Nemo > Monsters Inc.
Of course, this is an unfair poll, because they all rock. It's like picking a favorite child - it just couldn't (and shouldn't) be done! 🙂