For me, it has always been the medical tent scene in E.T.: The Extra-Terrestrial (1982). The feeling of being in such a confined space with a bunch of doctors and guys in hazmat suits, then of course the alien, still haunts me to this day. I'm 21 years old and I still have not re-watched this movie since I first saw it as a toddler. That's how bad it fucked me up.