Haven't seen too many, but I really liked the ones I've watched, and definitely intend to see more. I've seen Spirited Away (probably my first), My Neighbor Totoro, Ponyo, Kiki's Delivery Service and Princess Mononoke, and Spirited Away is probably my favorite but it's tough to say since I really loved all of them.
I love that these films don't have to be super deep (Totoro, Ponyo and Kiki's are just super cute and charming) to tell a great story, but they can also delve into more interesting and thoughtful subjects, like they do in Spirited Away and Mononoke.