Good question. It's hard to single one out, because several have had some impact on me, Harold and Maude, You Can't Take it With You, Thelma & Louise, etc.
I think The Bat from 1959, I first saw that at either 2 or 3 years old...and it's not extreme like today's horror movies...but it's no kids' movie either: there's murder, people being shot, getting their throats ripped open, and a lot of stuff that's just going to go over your head at that age about bank robbery and embezzlement, and as a kid that movie scared me to death, but I kept going back to it until I wore the tape out.
It was what got me interested in mysteries, it was my first experience of strong women on the screen with the characters of Cornelia Van Gorder and especially her maid, Lizzie Allen...that was THE movie where I first learned to appreciate the film's climax, it was so good, it had that much impact on me, and it's still one of my favorite movies; and it was my love of this movie, that led me to see the previous versions, and finally to seek out the novel it started from, which in turn was responsible in part for how I became as big of a reader as I was, because that was the first thing I sought out when I got my library card, The Bat by Mary Roberts Rinehart, and then I started reading her other books, and from there other mysteries, and came out of my very picky reader shell...so looking at it that way I would say this is the one that's had the most impact on me.