The whole LOTR series.
IMO didn't capture the spirit of the books at all (I'm not even a Tolkien fanatic, and I can still see that). I'm fully aware that it's impossible to cram the whole work into ~ 9 hours, but book to movie adaptations is all about capturing the essence of the work, in that Jackson completely fails.
Titanic.
Incredible that that piece of sappy calculated crap is from the creator of such films as T2, the Abyss, True Lies and Aliens.
People in 1996 must have been really desperate for entertainment.