re: What was that one movie that just blew you away when you saw it?

Tombstone
Alien
Halloween
Patton
LOTR
Jaws
Romeo and Juliette
Psycho
Casablanca
Star Wars

My definition of "blown away" is not limited to but includes elements of: story, casting, acting, genre, memorable moments, evokes emotional response, presentation via photography and special effects, personal taste, venue of original viewing, my age/maturity level at first screening, other intangibles.

Matrix
Inception

The first two that came to mind for me were Ex Machina and The Prestige. Both had that twist that made me rewatch them within a week. Both I constantly told friends they had to see.

Have to go with Fandango.

My dad had seen it on cable and told me "the main guy in this film is going to be a huge star". That guy was, of course, Kevin Costner. Up to that time, the only thing was known for was being the corpse in the opening credits of The Big Chill.

IMHO, it's about as close to perfect an example of filmmaking, from original premise to screenplay to casting to execution, as I've ever seen. And other than my father's endorsement, I knew absolutely nothing about it going in. And the fact that it had an indie budget and zero marketing makes it even more impressive.

Almost forgot about this one.

Forrest Gump.

Remember going to see it opening weekend. Summer after my senior year and the last summer I spent with my high school friends. Spent the night at a friends house and we decided to go see a movie that night. Didn't see anything else interesting we had not already seen so we took a chance on this one not really knowing anything else about it. Remember leaving the theater thinking it was one of the best movies I had ever seen which was completely strange for a action/scifi/fantasy loving geek. Sad thing about modern times and the information age we never get to walk into a theater with zero preconceptions of a movie any more. Now it's all information and expectations before seeing the movie.

Couple of good ones in here. Lord of the Rings was memorable especially.

I would add Crouching Tiger, Hidden Dragon.

Great movies listed already:

LOTR-Huge Tolkien geek was pumped for this while it was being made and still did not disappoint.

Jurrasic Park-Remember watching thinking, there is nothing they cannot do in the movies now.

Saving Private Ryan-WW II geek of the highest order, just blew me away with it's intensity.

Borat-Have never laughed so hard at a movie, literally could not breath I was laughing so hard. helped it was opening night and the crowd was into it as well.

Titanic when I was 10 years old.

The second half sinking scene was something to behold at the time.

Sixth Sense

The Game

Unforgiven