Jeepers Creepers for me to.
Was really enjoying the film but when you finally see the monster I laughed and all the horror of the film soon sapped away
I like the whole of JC but I agree that the first part is stronger than the second.
Knowing is another good example. It leaves you with the feeling that you've watched a bad film because it totally loses credibility at the end but the truth is that the first half is really good.
I would have to say War of the Worlds too. The opening is brilliant. Up there with the best openings to a film I've seen. It does lose its way a bit though and the ending is just silly.