I'd really have to say the anime that are philosophical. I mean, Princess Mononoke is incredible and it MEANS something: it's an impressive insight at humanity. Death Note is just completely off the wall wicked. It's always trying to make you think ahead or just admire how friggin' smart the creators must be. Finally: Full Metal Alchemist. That show for me was a lot more violent and gruesome than I thought it would be but it used the graphic violence to really show expression. The human characters helped a lot too.
Looking back: the Philosophical animes are the only ones that you can actually remember what happened in and they're the ones that keep you hooked.