For me it was 'Willow.' A 1988 film that I had been jumping out and down since I saw the first promotion, well over a year earlier. I remember my brother and I were excited because of George Lucas, creator of the 'Star Wars' involvement. We talked about how great it would be, changed the world, etc.
Made it a special event opening, but was very disappointed. Very disappointing, so much that I didn't get excited about movies for many years to follow. I finally saw it again with some friends a few years ago. It still was an unfulfilled experience.