I was quite impressed with the trailer for Transformers, so it gets my vote.

Harry Potter - read the book, the film will be crap in comparison with a 20 year old kid trying to be 14 or whatever.

Die Hard - will probably be as bad as Terminator 3.

Spiderman 3 - I get the impression that most of the story line has been given away by the trailers we've seen; although I'm sure we'll watch it eventually, I suspect the concept may be wearing a little thin (there aren't many films that can successfully manage 2 sequels).