I've yet to see Howl's, but Mononoke and Spirited Away are both VERY good films, and I strongly recommend both. Hayao Miyazaki is different in anime in that he tends to cut down on adult content in favor of character development, internal conflict, and emotion. Wonderful movies, he makes, wonderful!
Mononoke even has a bit of slice and dice action for he who gets bored of the movie's awesome characters.
