Harry Potter and the Goblet of Fire
It's rather admirable.
This is certainly the darkest chapter so far. Harry having to go through a death tournament, and ultimately contributing to the revival of his greatest enemy, Voldemort? Not to mention, Cedric Diggory's death marks the first death that has a real impact.
Since the magic cannot be reversed, Harry is forced to go with it and brave three exceedingly difficult tasks.
Its hard to pick which potter film is the least best.