Star Trek III: The Search for Spock. Unlike many my age, I hadn't been into Trek during the rerun heyday of the 70s. I had caught TMP on Showtime and it held my interest...hadn't seen TWOK at all yet. I went to see TSFS with a junior high school friend who was into Trek. I made a point of checking out TWOK on home video that summer, and this proved to be a major stepping stone in my becoming a fan.
Even not knowing much about Trek at that point, I got a good laugh out of Bones' reaction upon finding out what Spock had done to him....
Finally a kindred spirit.
I should mention as an epilogue to my story that by the time STIV rolled around, I saw it in the theater four times. The only other movie I can recall having seen four times in first run was Batman in '89.
Of course, this owes partly to how old I was when these films came out...if I'd had my own money and transportation when the Star Wars OT came out, I would have seen those films a crazy number of times first-run.
(I did manage to see ANH and, IIRC, TESB each four times in the theater thanks to theatrical rereleases, not counting the Special Editions...as well as ROTLA for the same reason. One of my fonder memories from early adolescence is of trekking on foot on hot summer days to a reasonably nearby dollar matinee theater that would get these films. Those classic films on the big screen before they were available on home video, plus the air conditioning--Pure bliss.)