I don't know that I have been to that many national parks or forests (maybe 2 or 3).  I have backpacked in one national forest.  I don't think it gets a lot of visitors and it seems to get really trashed.  So, I doubt I would put it high up on lists.  I guess one that I can remember (if it was a national park back then) a little bit about is Carlsbad Caverns.  I guess that would be the one.