Well, having come home from my third trip in two days, I have to say UP ranks way up there for me! It is one I would recommend to anyone and everyone, and I don't often recommend movies (even ones I personally like).
This is too hard, though! I have to say, honestly, that I think the first two Toy Story movies, taken as a pair, are "practically perfect in every way," (I'm trusting TS3 to be a worthy successor), and I never ever get tired of the last 20 minutes of Ratatouille (from the point where Remy goes back to the restaurant to save the day), and I still have a big ol' sentimental attachment to "A Bug's Life." I have to say, though, I've never met a Pixar movie I didn't like.