Nice list although I think that the true underrated gems are foreign animated films that had international releases like Persepolis and Triplets of Belleville. Also I agree with you on The Iron Giants, great film but poorly handled release date/ marketing.
Doomroar
??? what? those movies are not underrated, if anything only The Secret of NIMH deserves to be here!
All the others are classics, with probably the exception being TMNT which is more of a cult classic, for real, it makes me wonder, if this were a top 6 you probably would have put Atlantis: The Lost Empire here.
rubberonion
outside of the animation bubble, black cauldron is not considered a classic... nor is treasure planet. whether it failed at the box office, was passed over critically (hence posting the metrics I did), or just buried in history by other films (like secret of nimh) - these all qualify as underrated. did you read the post and disagree with what I said? or did you look at the films and disagree with the choices themselves because I think I made my case well enough in the content of the post. thanks for the comment!