This is another category with a legion of answers, but I'm going to choose...and here I'm pausing while I try to actually choose...The Empire Strikes Back. Most of my all time favorite films are such a part of my life experience that I have trouble remembering the time before they were ingrained in my consciousness, but I remember going to the opening of Empire with Duke, and Targon, and Turtle, and Rico very clearly. Well, almost very clearly. I know I tried my best to dress like Han Solo (and I thank all the gods of technology that digital cameras were still decades away at that point) but I don't recall if anyone else in our group did. The cinema we went to that day, the Plaza Twin, I believe, is now a car dealership. The day was sunny, and not too hot for a South Florida May, and it stands out in my memory as one of those halcyon days of youth when a group of friends, totally unconcerned with a future that would scatter them hither and yon all too soon, took part in what would become a bit of pop culture history. We watched in rapt enthusiasm and afterward debated whether Vader was lying or not, and tried to mess with the folks waiting in line for the next showing by lamenting the death of Luke.(although none of us could keep a straight face) I don't recall what we did after, or how we got there, got home, etc. But when I think of my group of friends, all but one of whom is still close and probably reading this, that is one of my favorite, and most vivid memories.
Oh, and I'm also glad none of them have any pictures of my post-Empire tee shirt I had made. ;-)