I'm not sure Close Encounters delivers a punch exactly, but it is an interesting fantasy take on a family falling apart. When viewed as that I can see it as powerful.
For me, it was when the aliens took the kid away from her mom and her reaction to it. When you're 4 years old and seeing that happen in a movie, I guess it can hit you pretty hard at that age.
The Fox & The Hound - I feel silly listing a cartoon, but the end worked well, & it built up tension well.
He stuck up for his buddy at the end. That's some deep loyalty, man. Like mob movie sh*t...you know, "never rat on your friends," that sort of thing.
Star Wars - Seeing Luke turn off his computer & seeing Han come back.
That, and all the other stuff that led up to that moment, like Ben's ghost calling out from the grave "Use the Force, Luke!" And yeah, the destruction of Alderaan and Ben Kenobi getting killed (this was a couple decades before I read Lord Of The Rings and experienced the death of Gandalf) is stuff that can traumatize a 3 year old; think of all the Xanax cases years later that resulted from the death of Mufasa in
The Lion King (1994).