It seems rare nowadays that any moment in a movie will truly blow your mind. In fact, I can't really think of any good examples this milennium. Inception was good, and came close, but there was no huge wow for me. Here are the movies I can think of that must have given the initial theater viewers a huge wow.
- Sixth Sense (1999) - The movie that put M. Night Shymalan on the map did so for good reason. The big reveal was artfully foreshadowed, and the big reveal at the movie's climax let loose quite the plot payload. Baboom. Your mind is blown.
- The Matrix (1999) - Until the Matrix's big reveal, it was a confusing mess. What the hell is going on here? After Morpheus lets on what's really happening, your mind can't assemble the pieces quickly enough. It all makes sense, but as a first-time viewer of the movie who's just caught on, it takes a while to even catch up. It all makes for one hell of a ride.
- Alien (1979) - In 1979, I was five years old - much too young for horror sci-fi. But I imagine that 1979 was a time before movie trailers began spoiling the best parts of movies. If you went to Alien on opening night before your friends had told you what happened, and had no idea what was coming before the alien popped out of that guy's chest, it must have been an amazing shock.
- Star Wars (1977) - Perhaps it's not as grand a surprise as the others on this list, but seeing the opening shot of a star destroyer scrolling by must have really been something. You keep expecting the ship to end, and it just gets larger and larger. The effects alone were amazing in 1977. Alas, I was three years old, and wouldn't see a Star Wars movie in the theater until Empire came out years later.
- The Wizard of Oz (1939) - Now this one's really going back, but I've heard tales of moviegoers who attended a screening of The Wizard of Oz in the theater before they'd ever seen a colorized motion picture before. The fact that the first 15-20 minutes of the movie are in black & white only adds to the dramatic impact when dorothy enters the suddenly colorized world of Oz. I've heard of people being moved to tears by the experience. It just sounds kind of cool.