The opening scene from A New Hope with the large Tantive IV being dwarfed by the immense Star Destroyer, passing overhead and the rumble of the surround sound is one of the best openings to a film for me.
Se7en - Purely for the audience's collective reaction to seeing that the 'Sloth guy' isn't actually dead.
The first shot of the AT-AT's foot followed by the pull back in ESB.
The Stay Puft sequence of Ghostbusters.
The fight scene in Army of Darkness.