Spirited Away and Totoro have grown on me. I didn't like them as much when I first saw them as when I saw Castle in the Sky, but the more I learn about the stories (and or catch things on rewatching) the better I find them to be. So, to me, they have better re-watchability than Castle in the Sky.
Spirited Away, in particular, catapulted itself into my #2 spot once I discovered what the "story" was really about, and I was just blown away with how well Miyazaki told it.
That being said, Mononoke has a rather one dimensional story. It's just that I adore the characters and the visuals so much, and it was the first Miyazaki movie that I ever saw (which, is pretty common).
Here's another anime I enjoy (and this is odd for a guy). Inuyasha.
It's bad, but it's so bad, that I think it's good! I can easily waste an hour or two watching a few episodes.