I would put Toy Story, Toy Story 2, Finding Nemo, and Monster's Inc in my personal top 150. Ratatouille, A Bug's Life, and The Incredibles just didn't work quite as well for me. I haven't seen Cars yet, but I know so many kids that are still crazy about it.
The thing that Pixar movies do so well is creating interesting, lovable, visually stunning characters that adults can relate to, while their kids are having pure fun. Sulley from Monsters Inc. would have to be my favorite.