A lot of my favourites have already been mentioned (my top picks being The Dark Knight and Cloverfield campaigns), but a couple that instantly spring to mind were made for two different epic fantasy films, both looking to be the successors to the Lord of the Rings, both based on seminal pieces of children's literature, both of the trailers sent my pulse rate soaring, and both films ended up sucking major balls to varying degrees of suckiness.
http://www.youtube.com/watch?v=ntDg19AeK20
I'm still particularly bitter about how much they royally cocked up The Golden Compass; I'm holding out hope someone else will tackle it in my lifetime.