Paper Man, the animated short they showed before Wreck-It Ralph, I thought was really good. Best I could find on YouTube was a crappy video taken inside the theater though, and it doesn't do it justice. Just make sure you see it if you get the chance, the score is equally amazing.
Everything Pixar does is great. Paper Man was wonderful. Flying Books of... from earlier in this thread is a total winner. And classic stuff from Warner Brothers, Disney, Tex Avery, UPA and George Pal will always be favorites.