Having rewatched Spirited Away and My Neighbor Totoro recently, I actually now think Totoro is his best work, at least from those I've seen (those two, Mononoke, Castle in the Sky, Moving Castle) . I thought it was a bit too light when I first saw it a few years ago, but rewatching it it was simply perfect. It reminded me of Le Petit Prince, in that what's great about that book is how enchanting it is to read as a child, and how interestnig it is to rediscover as an adult. I would say the same of My Neighbor Totoro, whereas I don't think Spirited Away and Mononoke work as well for children. They're also longer, and I've come to really appreciate brevity in movies.