No doubt: Gremlins.
I watched it (when I was still at the tender age of 5) around its release at the theater.... it alone caused my fear of the dark! After viewing it, I always slept with the covers pulled up over my head (as if that would deter beasties! 😉 ), and never dared to get up from bed, or move from whatever spot I happened to be, in the dark.
Sure, it's a bit of a dark comedy now. I laugh. But it was a long nightmare then. 🙁
The fear is quite cured now, fortunately, but the movie (and those creatures) left their mark. 😀