I would say that LOTR: ROTK has the worst differences in book vs. movie. Gollum actually manipulating Frodo into telling Sam to go home? That was the worst. Because Sam would never and they face Shelob together. I mean imagine you make it to Minas Morgul and travel around with Frodo and he just says:"Go home". I felt that was a bit stupid and given that the characters looked like teenagers, Frodo often comes off as way too whiny.