I can't pick one, so I'll list my favourites; Toy Story 1 & 2, The Lion King, Akira, Batman: Mask of the Phantasm and Batman: Under the Red Hood.
EDIT: I've yet to watch Toy Story 3, How to Train Your Dragon, A Nightmare Before Christmas or some of the classic Japanese anime movies (save for Akira), but I've heard that they're all great. I thought that the first 30 minutes of Wall-E were boring, so I didn't watch the rest of it. I'll give it another chance soon.