I think the best one isn't a difficult question: for me it's WALL-E. The way they made the film, the feel of the movie, the music (keeps repeating days after days in my head), the first half almost without any dialogue. After the humans enter the movie, I like the more simplistic way the humans are shown. The robots have so much more detail, character even.

The worst film pixar made, is "A Bug's Life". It fails the comparison with Antz at the moment it came out. To me it felt like cheap clone. The story didn't interest me at all. I must admit I was a little boy back then, but still, I can't fight my memory.

As a side-node, I only seem to buy dvd's of animated movies. The last one being Ice Age 3 (still need to watch him, since it's a gift for the holidays :)