What did you think were the worst and best changes that the movies made (LOTR or Hobbit, either of them)?
For me the worst change was probably the ignorance of the Ents.
The best... that's a bit more tricky, but I think the Paths of the Dead were a bit too 'meh' and confusing and anticlimactic in the book. I like that they made them a bit more exiting (though I think they should have kept them a bit more accurate)