Star Wars, A New Hope.
Saw it as a teenager, now watch it with my children. It's the film that I think for my generation has been the backdrop of our lives. It's a shared experience for all of us who sat in a cinema when we were young, with ours mouths open, feeling let alone hearing and seeing, the opening scene with the battlecruiser flying across the screen.
I know it's not the best film ever made, it's not even the best in the trilogy, but no film for me, has ever come close to recreating the feeling of awe and excitement.
To quote Simon Pegg in Spaced "You had to have been there".