As a general rule, I've liked most if not all of the Pixar movies I've seen, from Finding Nemo to Inside Out there hasn't been a movie that I didn't like. But to pick a favorite would be hard, like picking a favorite child. But if I had to chose I would have to say it would be Wall E. The story itself plays to a fear for the future we all have if we don't care for our planet, and then there's the cute love story between two robots. Wall E is also very human like in that he is inquisitive and likes to collect things. The way Pixar made his eyes droopy like a sad and lonely puppy, they made them looks like real eyes, he's a very relatable character.
If I had to pick my least favorite movie, it would probably have to be A Bug's Life, the movies itself was good no doubt; and the animation for the time was pretty solid. But the fact that it was one of the first Pixar movies I'd ever seen, is possibly my least favorite. It's what I would compare all future Pixar movies too, and every movie afterwards would fly past the bar.